modules/gnome: Fix unset install_dir

install_dir needs to be set to something, because CustomTarget expects
that. genmarshal still relies on CustomTarget setting it internally
itself.

Fixes #9350

mesonbuild/modules/gnome.py

@@ -1639,7 +1639,7 @@ G_END_DECLS'''
                 raise MesonException(f'Genmarshal does not take a {arg} keyword argument.')
 
         install_header = kwargs.pop('install_header', False)
-        install_dir = kwargs.pop('install_dir', None)
+        install_dir = kwargs.pop('install_dir', [])
 
         custom_kwargs = {
             'input': sources,
@@ -1664,8 +1664,7 @@ G_END_DECLS'''
         body = build.CustomTarget(output + '_c', state.subdir, state.subproject, custom_kwargs)
 
         custom_kwargs['install'] = install_header
-        if install_dir is not None:
-            custom_kwargs['install_dir'] = install_dir
+        custom_kwargs['install_dir'] = install_dir
         if new_genmarshal:
             cmd += ['--pragma-once']
         custom_kwargs['command'] = cmd + ['--header', '@INPUT@']