tools/settings_migrate_group: skip the enum stage when every row is a reference

An all-reference migration owns no enum members; the marker splice
would find nothing to replace.

Author: libretroadmin

tools/settings_migrate_group.py

@@ -697,6 +697,9 @@ def _plains(text):
         print("  enum stage skipped: %s guard mismatch enum=%s def=%s" % (_T, sorted(_et), sorted(_dt)))
         _enum_ok = False; break
 
+if not enum_rows:
+    _enum_ok = False
+    print('gate: enum stage - nothing to consolidate (all rows are references)')
 if _enum_ok:
     MARK = '/*__SETTINGS_DEF_ENUM_MARK__*/'
     _first = True