bitbake/fetch2/git: Don't set the branch name to None, that makes no sense

With floating revisions and no specified branch, the fetcher could fail
with some obtuse errors. This was due to the branch name being set to None
which makes no sense. This patch reworks some conditions to avoid this.

(Bitbake rev: 740c58d43cfb1445dd126e4827bb70ce988ca107)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/fetch2/git.py

@@ -123,7 +123,8 @@ class Git(FetchMethod):
         for name in ud.names:
             # Ensure anything that doesn't look like a sha256 checksum/revision is translated into one
             if not ud.revisions[name] or len(ud.revisions[name]) != 40  or (False in [c in "abcdef0123456789" for c in ud.revisions[name]]):
-                ud.branches[name] = ud.revisions[name]
+                if ud.revisions[name]:
+                    ud.branches[name] = ud.revisions[name]
                 ud.revisions[name] = self.latest_revision(ud.url, ud, d, name)
 
         gitsrcname = '%s%s' % (ud.host.replace(':','.'), ud.path.replace('/', '.'))