Fix building without writing a source map

Reviewed By: bestander

Differential Revision: D3690117

fbshipit-source-id: f162352c09c4c2a387a9414dbd7da55b86551ee3

Author: davidaurelio

local-cli/bundle/buildBundle.js

@@ -28,7 +28,7 @@ function buildBundle(args, config, output = outputBundle, packagerInstance) {
 
   const requestOpts = {
     entryFile: args.entryFile,
-    sourceMapUrl: path.basename(args.sourcemapOutput),
+    sourceMapUrl: args.sourcemapOutput && path.basename(args.sourcemapOutput),
     dev: args.dev,
     minify: !args.dev,
     platform: args.platform,