fix gyp to work on macOS without XCode

Most builds are possible with just the
"Command Line Tools for XCode"
nodejs has extensive experience with this scenario

BUG=gyp:477

nodejs PR-URL: nodejs/node#1325
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

pylib/gyp/xcode_emulation.py

@@ -1662,6 +1662,8 @@ def _GetXcodeEnv(xcode_settings, built_products_dir, srcroot, configuration,
     sdk_root = xcode_settings._SdkRoot(configuration)
     if not sdk_root:
       sdk_root = xcode_settings._XcodeSdkPath('')
+    if sdk_root is None:
+      sdk_root = ''
     env['SDKROOT'] = sdk_root
 
   if not additional_settings: