Notification icons: Render empty string for "custom" setting

What does this MR do?

In !40709 (merged) we update the notification icons to use our internal Gitlab SVGs.

However, when a user selects the Custom notification setting, the sprite_icon helper responds with the following error as it cannot find an icon for the custom level:

is not a known icon in @gitlab-org/gitlab-svg

This MR ensures that an empty string gets rendered in this case to prevent the "missing icon" error from showing up.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #254224 (closed)