Getting cryptic error if the file to return ENV object return null or {} instead

I have solved the issue myself but I thought it can be better.

It is good that it throws an error about it but the cryptic error message like this is not helping
```
remote: Module build failed: TypeError: /tmp/build_43fb00b7ca544651b6eb4730c0d0ad94/components/BuildTeam.js: Cannot convert undefined or null to object
remote: at PluginPass.UnaryExpression (/tmp/build_43fb00b7ca544651b6eb4730c0d0ad94/node_modules/babel-plugin-transform-define/lib/index.js:34:27)
remote: at newFn (/tmp/build_43fb00b7ca544651b6eb4730c0d0ad94/node_modules/babel-traverse/lib/visitors.js:276:21)
remote: at NodePath._call (/tmp/build_43fb00b7ca544651b6eb4730c0d0ad94/node_modules/babel-traverse/lib/path/context.js:76:18)
remote: at NodePath.call (/tmp/build_43fb00b7ca544651b6eb4730c0d0ad94/node_modules/babel-traverse/lib/path/context.js:48:17)
remote: at NodePath.visit (/tmp/build_43fb00b7ca544651b6eb4730c0d0ad94/node_modules/babel-traverse/lib/path/context.js:105:12)
remote: at TraversalContext.visitQueue (/tmp/build_43fb00b7ca544651b6eb4730c0d0ad94/node_modules/babel-traverse/lib/context.js:150:16)
remote: at TraversalContext.visitSingle (/tmp/build_43fb00b7ca544651b6eb4730c0d0ad94/node_modules/babel-traverse/lib/context.js:108:19)
remote: at TraversalContext.visit (/tmp/build_43fb00b7ca544651b6eb4730c0d0ad94/node_modules/babel-traverse/lib/context.js:192:19)
remote: at Function.traverse.node (/tmp/build_43fb00b7ca544651b6eb4730c0d0ad94/node_modules/babel-traverse/lib/index.js:114:17)
remote: at NodePath.visit (/tmp/build_43fb00b7ca544651b6eb4730c0d0ad94/node_modules/babel-traverse/lib/path/context.js:115:19)
```
This throws when the page it is saying at fault does not even use any of the variables supposed to be defined in env, but simply uses Unary Expression like "typeof".

Perhaps make the error clearer if the env passed in .babelrc is returning {} or null?

Thanks

.babelrc の ENV 設定と babel-plugin-transform-define/lib/index.js の失敗するパスを確認することから始めます。ENV が null または {} を返すケースを、typeof のような単項式だけを使用するファイルも含めて再現します; TypeError の不可解な内容を露出させるのではなく、ENV の値が無効であると失敗が報告すれば完了です。

索引モデルが issue の本文から書いたものです。