Don't allow empty string as first argument of action.

test/rules-dirname/src/subdir/input-rule-dirname.gyp

 # Copyright (c) 2011 Google Inc. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'targets': [
     {
       'target_name': 'print_rule_input_path',
       'type': 'none',
       'msvs_cygwin_shell': 0,
@@ skipping 64 matching lines @@
               'action_name': 'setup_mount',
               'msvs_cygwin_shell': 0,
               'inputs': [
                 '../../../../../../<(DEPTH)/third_party/cygwin/setup_mount.bat',
               ],
               # Visual Studio requires an output file, or else the
               # custom build step won't run.
               'outputs': [
                 '<(INTERMEDIATE_DIR)/_always_run_setup_mount.marker',
               ],
-              'action': ['', '<@(_inputs)'],
+              'action': ['<@(_inputs)'],
             },
           ],
         },
       ],
     }],
   ],
 }