Fix failOnHint for unused exports

[tjs-intel]

unusedExportMap is a map so the Object.keys function should be used to check if it is empty.

[tjs-intel]

Confirmed that this does terminate the compilation with unused exports and the failOnHint: true.

[MQuy]

thanks @tjs-intel for fixing the issue 💯

[cmrd-senya]

I does not work for webpack 4 now with the latest version. Is the fix in this thread only function for webpack 5?

[tjs-intel]

The last version was released in 2021. I think the maintainer isn't releasing new versions.