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

Beginnen Sie mit der Überprüfung der .babelrc-ENV-Konfiguration und des fehlerhaften Pfads in babel-plugin-transform-define/lib/index.js. Reproduzieren Sie den Fall, in dem ENV null oder {} zurückgibt, einschließlich einer Datei, die nur einen unären Ausdruck wie typeof verwendet; die Aufgabe ist abgeschlossen, wenn der Fehler meldet, dass der ENV-Wert ungültig ist, anstatt den kryptischen TypeError offenzulegen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.