Non-repo explicit projects (plain folders) get a main-checkout lane whose
label is the folder basename, not a branch. Clicking "+" (new session) on
such a lane calls switchBranchInRepo -> switchBranch, which sanitizes the
basename to "" and throws "Branch name is required.", aborting the
session creation. Short-circuit switchBranch for roots that are not git
work trees so callers proceed with a plain session.
Fixes#83028
Inspired-atm
db1d8983e5