Set subparsers in build script as required

This fixes an error when invoking the build script without arguments.

Author: fwcd

build-script.py

@@ -887,7 +887,7 @@ def add_generate_source_code_args(parser: argparse.ArgumentParser) -> None:
     # -------------------------------------------------------------------------
     # Shared arguments
 
-    sub_parsers = parser.add_subparsers()
+    sub_parsers = parser.add_subparsers(required=True)
 
     # -------------------------------------------------------------------------
     generate_xcodeproj_parser = sub_parsers.add_parser(