Fix bug in ninja generator when there's no default action.

pylib/gyp/generator/scons.py

Index: pylib/gyp/generator/scons.py
diff --git a/pylib/gyp/generator/scons.py b/pylib/gyp/generator/scons.py
index 7ac685f1bcfa134235439909a16d4de8517b8b61..9a4a8180e249961885b350835cc8c81106b86f81 100644
--- a/pylib/gyp/generator/scons.py
+++ b/pylib/gyp/generator/scons.py
@@ -464,7 +464,6 @@ def GenerateSConscript(output_filename, spec, build_file, build_file_data):
   rules = spec.get('rules', [])
   for rule in rules:
     name = rule['rule_name'].translate(string.maketrans(' ()-', '____'))
-    a = ['cd', src_subdir, '&&'] + rule['action']
     message = rule.get('message')
     if message:
         message = repr(message)
@@ -474,6 +473,10 @@ def GenerateSConscript(output_filename, spec, build_file, build_file_data):
       poas_line = '_processed_input_files.append(infile)'
     inputs = [FixPath(f, src_subdir_) for f in rule.get('inputs', [])]
     outputs = [FixPath(f, src_subdir_) for f in rule.get('outputs', [])]
+    # Skip a rule with no action and no inputs.
+    if 'action' not in rule and not rule.get('rule_sources', []):
+      continue
+    a = ['cd', src_subdir, '&&'] + rule['action']
     fp.write(_rule_template % {
                  'inputs' : pprint.pformat(inputs),
                  'outputs' : pprint.pformat(outputs),