Handle the case where transformSync returns null (#264)

Summary:
**Summary**

Fixes a bug when ignoring a file in the babel config: #255 (comment)

**Test plan**

Couldn't find any obvious tests that test this part of the code. Happy to add some tests if someone can point me in the right direction.
Pull Request resolved: #264

Differential Revision: D10063236

Pulled By: rafeca

fbshipit-source-id: 04205b856c513fd90c0ae022c2cbb4deeef0c993

packages/metro/src/reactNativeTransformer.js

@@ -144,14 +144,19 @@ function transform({filename, options, src, plugins}: BabelTransformerArgs) {
 
   try {
     const babelConfig = buildBabelConfig(filename, options, plugins);
-    const {ast} = transformSync(src, {
+    const result = transformSync(src, {
       // ES modules require sourceType='module' but OSS may not always want that
       sourceType: 'unambiguous',
       ...babelConfig,
       ast: true,
     });
 
-    return {ast};
+    // The result from `transformSync` can be null (if the file is ignored)
+    if (!result) {
+      return {ast: null};
+    }
+
+    return {ast: result.ast};
   } finally {
     process.env.BABEL_ENV = OLD_BABEL_ENV;
   }