fix: NextJS 12 Error

[Aviortheking]

With NextJS 12 there is an error when we start the dev server with the plugin

TypeError: Cannot use 'in' operator to search for 'not' in undefined
    at /home/avior/projects/documentation/node_modules/next-pre-css/index.js:54:12
    at Array.reduce (<anonymous>)
    at /home/avior/projects/documentation/node_modules/next-pre-css/index.js:108:36
    at Array.map (<anonymous>)
    at Object.webpack (/home/avior/projects/documentation/node_modules/next-pre-css/index.js:107:37)
    at Object.getBaseWebpackConfig [as default] (/home/avior/projects/documentation/node_modules/next/dist/build/webpack-config.js:1238:32)
    at async Promise.all (index 0)
    at async Span.traceAsyncFn (/home/avior/projects/documentation/node_modules/next/dist/trace/trace.js:79:20)
    at async Span.traceAsyncFn (/home/avior/projects/documentation/node_modules/next/dist/trace/trace.js:79:20)
    at async HotReloader.start (/home/avior/projects/documentation/node_modules/next/dist/server/dev/hot-reloader.js:328:30)

I fixed it by adding checks to see if the value is undefined or not