Show the default "default branch" in the API for empty projects

What does this MR do?

Where someone has configured a default branch, it's confusing for the API to return nil for empty projects here. It also makes it difficult for them to build automation around arbitrary default branches.

Returning the expected default branch name should resolve these issues.

This change only affects projects with no repository, or an empty repository. Currently we return nil in those cases, but it's easy to return the value that the default branch would be, if it exists, since we store that in the database (a value like main, master, or develop, for instance. It may be configured at group or instance level). It's accessible as project.default_branch_or_main.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #330622 (closed)