Default avatar not shown when Gravatar disabled

Summary

The user avatars are not rendered properly in the Protected Branches (and other) dropdown(s) in the project settings (see the attached screenshot). For users who have no avatar uploaded a "null" string is inserted as the IMG SRC instead of a default image.

This is happening on a self-hosted instance with Gravatar disabled.

Steps to reproduce

1. Disable Gravatar under Settings/General/Account and limit
2. Create a new user
3. Assign the user permissions on a project
4. Navigate to the project settings and under Repository/Protected Branches open one of the dropdown listing users
5. The user will appear with a "broken" avatar image

What is the current bug behavior?

For users with avatar uploaded a "null" string is inserted as the IMG SRC instead of a default image in the dropdown.

What is the expected correct behavior?

Default image show be used as in other places across the application already.

Relevant logs and/or screenshots

See screenshot attached

Output of checks

(If you are reporting a bug on GitLab.com, write: This bug happens on GitLab.com)

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:		
Proxy:		no
Current User:	git
Using RVM:	no
Ruby Version:	2.6.6p146
Gem Version:	2.7.10
Bundler Version:1.17.3
Rake Version:	12.3.3
Redis Version:	5.0.9
Git Version:	2.27.0
Sidekiq Version:5.2.9
Go Version:	go1.13.11 linux/amd64

GitLab information
Version:	13.2.2-ee
Revision:	618883a1f9d
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	PostgreSQL
DB Version:	11.7
URL:		https://gitlab.example.com
HTTP Clone URL:	https://gitlab.example.com/some-group/some-project.git
SSH Clone URL:	git@gitlab.example.com:some-group/some-project.git
Elasticsearch:	no
Geo:		no
Using LDAP:	yes
Using Omniauth:	yes
Omniauth Providers: 

GitLab Shell
Version:	13.3.0
Repository storage paths:
- default: 	/gitlab-data-git/repositories
GitLab Shell path:		/opt/gitlab/embedded/service/gitlab-shell
Git:		/opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check

Checking GitLab subtasks ...
Checking GitLab Shell ...
GitLab Shell: ... GitLab Shell version >= 13.3.0 ? ... OK (13.3.0)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Internal API available: OK
Redis available via internal API: OK
gitlab-shell self-check successful
Checking GitLab Shell ... Finished
Checking Gitaly ...
Gitaly: ... default ... OK
Checking Gitaly ... Finished
Checking Sidekiq ...
Sidekiq: ... Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Checking Incoming Email ...
Incoming Email: ... Reply by email is disabled in config/gitlab.yml
Checking Incoming Email ... Finished
Checking LDAP ...
LDAP: ... Server: ldapmain
LDAP authentication... Anonymous. No bind_dn or password configured
LDAP users with access to your GitLab server (only showing the first 100 results)
User output sanitized. Found 100 users of 100 limit.
Checking LDAP ... Finished
Checking GitLab App ...
Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... yes
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
5/3 ... yes
10/4 ... yes
9/5 ... yes
15/6 ... yes
2/8 ... yes
21/9 ... yes
22/10 ... yes
22/11 ... yes
21/12 ... yes
28/13 ... yes
29/14 ... yes
23/15 ... yes
23/16 ... yes
38/18 ... yes
42/20 ... yes
42/21 ... yes
42/22 ... yes
42/23 ... yes
42/25 ... yes
44/26 ... yes
44/27 ... yes
39/28 ... yes
6/29 ... yes
47/30 ... yes
47/31 ... yes
50/32 ... yes
50/33 ... yes
50/34 ... yes
51/35 ... yes
52/36 ... yes
55/37 ... yes
56/38 ... yes
57/39 ... yes
58/40 ... yes
59/41 ... yes
53/42 ... yes
53/43 ... yes
53/44 ... yes
62/45 ... yes
63/46 ... yes
64/47 ... yes
65/48 ... yes
65/49 ... yes
2/50 ... yes
39/51 ... yes
69/52 ... yes
71/53 ... yes
72/54 ... yes
72/55 ... yes
74/56 ... yes
74/57 ... yes
77/58 ... yes
80/59 ... yes
80/60 ... yes
80/61 ... yes
80/62 ... yes
80/63 ... yes
80/64 ... yes
80/65 ... yes
80/66 ... yes
81/67 ... yes
39/68 ... yes
83/69 ... yes
84/70 ... yes
85/71 ... yes
90/72 ... yes
3/73 ... yes
Redis version >= 4.0.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.6.6)
Git version >= 2.22.0 ? ... yes (2.27.0)
Git user has default SSH configuration? ... yes
Active users: ... 39
Is authorized keys file accessible? ... yes
GitLab configured to store new projects in hashed storage? ... yes
All projects are in hashed storage? ... yes
Elasticsearch version 6.x - 7.x? ... skipped (elasticsearch is disabled)
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished!

Possible fixes

N/A

Customer Ticket #167220(internal)