Make visualizer robust to graphs with NULL inputs.

test/cctest/cctest.gyp

 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 42 matching lines @@
         'compiler/graph-tester.h',
         'compiler/simplified-graph-builder.cc',
         'compiler/simplified-graph-builder.h',
         'compiler/test-basic-block-profiler.cc',
         'compiler/test-branch-combine.cc',
         'compiler/test-changes-lowering.cc',
         'compiler/test-codegen-deopt.cc',
         'compiler/test-control-reducer.cc',
         'compiler/test-gap-resolver.cc',
         'compiler/test-graph-reducer.cc',
+        'compiler/test-graph-visualizer.cc',
         'compiler/test-instruction.cc',
         'compiler/test-js-context-specialization.cc',
         'compiler/test-js-constant-cache.cc',
         'compiler/test-js-typed-lowering.cc',
         'compiler/test-linkage.cc',
         'compiler/test-loop-assignment-analysis.cc',
         'compiler/test-machine-operator-reducer.cc',
         'compiler/test-node-algorithm.cc',
         'compiler/test-node-cache.cc',
         'compiler/test-node.cc',
@@ skipping 228 matching lines @@
             '<@(_outputs)',
             'TEST',  # type
             'off',  # compression
             '<@(file_list)',
           ],
         }
       ],
     },
   ],
 }