gn: Employ the same INSERT_VARIABLE macro trick for switches.cc

tools/gn/variables.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/variables.h"
 
 namespace variables {
 
 // Built-in variables ----------------------------------------------------------
 
@@ skipping 942 matching lines @@
     "  Just these specific targets:\n"
     "    visibility = [ \":mything\", \"//foo:something_else\" ]\n"
     "\n"
     "  Any target in the current directory and any subdirectory thereof, plus\n"
     "  any targets in \"//bar/\" and any subdirectory thereof.\n"
     "    visibility = [ \"./*\", \"//bar/*\" ]\n";
 
 // -----------------------------------------------------------------------------
 
 VariableInfo::VariableInfo()
-    : help_short(NULL),
-      help(NULL) {
+    : help_short(""),
+      help("") {
 }
 
 VariableInfo::VariableInfo(const char* in_help_short, const char* in_help)
     : help_short(in_help_short),
       help(in_help) {
 }
 
 #define INSERT_VARIABLE(var) \
     info_map[k##var] = VariableInfo(k##var##_HelpShort, k##var##_Help);
 
@@ skipping 51 matching lines @@
     INSERT_VARIABLE(Sources)
     INSERT_VARIABLE(Testonly)
     INSERT_VARIABLE(Visibility)
   }
   return info_map;
 }
 
 #undef INSERT_VARIABLE
 
 }  // namespace variables