Set default for current_value in SAST config UI

What does this MR do?

This MR is related to this issue. In the current implementation of SAST config UI, we are reading the default_value from SAST template and current_value from .gitlab-ci.yml files. If there is no .gitlab-ci.yml file, the current_value is set to nil. In this issue, we are going to set the current_value to default_value if the .gitlab-ci.yml file is missing or a variable is not set in existing .gitlab-ci.yml.

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