Minor bug fixes for SastBuildActions

What does this MR do?

As a follow-on to Support writing variables to CI config, there were a few bugs that need to be fixed.

The stage parameter should be a string, not a symbol. The call to .compact doesn't remove empty strings, and we don't want to write variables to the YML that don't have a value set.

Since this is fixing things that are included in this release, I don't think this warrants a changelog entry.

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