Fix IDE on empty repo

What does this MR do?

This MR fixes the IDE assumption that the repo is not empty. Previously we did this at a top-level action, but this kept part of the Vuex state from being initialized. This MR takes that approach of letting the low-level actions (i.e. getBranchData) be more robust.

Screenshots

Before (on `master)	After (in this MR)
20200402_ide_empty_before	20200402_ide_empty_works

Follow-up issues

This MR fixes some issues, but there are still some preexisting issues to take care of before we can show the IDE button on an empty project.

• Add feature spec #213849 (this is currently blocked by another MR !27342 (merged))

Description	Screenshot
🐛 #213338 (closed) After the initial commit, the local state isn't updated properly so the default branch is treated as non-commitable. It works if you refresh the page.
🐛 #213581 (closed) If you visit the IDE for an empty project without being able to push code, we should probably show a helpful message explaining that you won't be able to do anything...

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

Closes #27915 (closed)