trail create: send no base when the repository names no default branch

When the local repository had no origin/HEAD, no main or master, trail create sent "main" as the Change's base anyway. A repository whose first push was a feature branch has no main, so the Change was stored on a base that doesn't exist: every push to it failed in ingest, and agents refused to start on it.

Send no base in that case. entire-api then uses the repository's default branch, or refuses a main that doesn't exist with base_branch_missing.

Trail: https://entire.com/et/entirehq/-/trails/1505