layerindex: allow empty Actual branch

Exposing the actual_branch field of LayerBranch in the UI
was intending to _enable_ setting a non-default value, but
the check was accidentally left in a state that _requires_
a value.

[YOCTO #15376]

Signed-off-by: Tim Orling <tim.orling@konsulko.com>

layerindex/forms.py

@@ -187,6 +187,7 @@ class EditLayerForm(StyledModelForm):
     def clean_actual_branch(self):
         import subprocess
         actual_branch = self.cleaned_data['actual_branch'].strip()
+        if len(actual_branch) > 0:
             process = subprocess.Popen(["git", "check-ref-format", "--branch", actual_branch])
             exit_status = process.wait()
             if exit_status != 0: