ArgumentNullException if WebRootPath not set

[LukeOwlclaw]

https://github.com/ligershark/WebOptimizer/blob/0c0d4df11b839dbb9207490afd872fc9242abee2/src/WebOptimizer.Core/Processors/RelativePathAdjuster.cs#L27

can cause

System.ArgumentNullException: 'Value cannot be null. '

According to https://stackoverflow.com/a/37988320/2484903 this can happen if wwwroot was removed.

To make WebOptimizer more robust, considering using env.WebRootPath ?? string.Empty