Fix a bug displaying project commit anchors

What does this MR do?

A repository can be non-empty, yet lack a root_ref. This may happen when non-standard git refs are pushed, for instance. When this is the case, we fail to generate a link to commits, resulting in a 500 error.

Using the "default" default branch in this case allows us to generate a link. It leads to a location that will 404, but that's a better outcome than the current scenario, and consistent with other links in this helper.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not 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.

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 #329862 (closed)