Fix failure with PO translation linter when there are no translations

What does this MR do?

When the HTML todolist contains a string that had no translations this method was producing an error as it tried to call .include? on a nil object. This was revealed during this pipeline run (https://gitlab.com/gitlab-org/gitlab/-/jobs/673958396) for the latest merge of Crowdin translations.

Fix this by checking for the presence of translations first.

Caused by: #217933 Errors appearing in: !38772 (merged)

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