fix(babel): Handle ignored files in babel v7 (#4393)

* fix(babel): Handle ignored files in babel v7

In babel v7 `babel-core.transform()` might return null if a file is ignored.
This makes babel-jest not throw a TypeError in this case.

* style: Remove trailing spaces

* Update index.js

packages/babel-jest/src/index.js

@@ -123,7 +123,9 @@ const createTransformer = (options: any) => {
         ]);
       }
 
-      return babelTransform(src, theseOptions).code;
+      // babel v7 might return null in the case when the file has been ignored.
+      const transformResult = babelTransform(src, theseOptions);
+      return transformResult ? transformResult.code : src;
     },
   };
 };