ign-3to2: Don't crash if there's nothing to merge
The previous change here broke the basic `cosa run` targeting RHCOS.
@@ -40,7 +40,7 @@ for f in ign['storage'].get('files', []):
f['contents'] = append[0]
# replace the merge keyword with append
-merge = ign['ignition'].pop('merge')
+merge = ign['ignition'].pop('merge', None)
if merge is not None:
ign['ignition']['append'] = merge
