Take into account empty strings in vulnerability reports vendor names

What does this MR do?

Sometimes the backend returns an empty string for some vendors in the security dashboards:

and our guardian function is simply checking whether there is any other vendor other than GitLab like:

hasAnyScannersOtherThanGitLab() {
  return this.filteredVulnerabilities.some(v => v.scanner?.vendor !== 'GitLab');
},

This MR adds an extra check for empty strings.

Screenshots

This MR prevents displaying the name GitLab as a vendor when it's the only one executed in the security reports. It fixes this bug:

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

Related to #267513 (closed)