Hide scanner filter vendor header when there's only the default vendor

What does this MR do?

On the vulnerability report, there is a scanner filter dropdown that filter the vulnerability list:

Currently, this dropdown always shows the vendor name (the dropdown item in bold) even if GitLab is the only shown vendor. This MR changes it so that if there are no custom vendors and GitLab is the only vendor, the header and the divider are hidden and the dropdown items are shown as a flat list. If there are custom vendors, the behavior is unchanged and the vendor header and divider are shown.

Master	This MR

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #330248 (closed)