Do not assume AppData and ProgramFiles exists in defaultParams
Do not assume AppData and ProgramFiles exists in defaultParams

This fixes calling OpenCover in Linux/MacOS environments where the
OpenCoverDefault value throws a ArgumentNullException because the '@@'
operator will call Path.Combine to ProgramFiles and AppData which do not
exist in Linux/MacOS environments.

This fix falls back to the behaviour where ExePath and TestRunnerExePath
must be passed explicitly in Mono environments running Linux/MacOS.

beeker/FAKE,beeker/FAKE,rflechner/FAKE,MichalDepta/FAKE,MichalDepta/FAKE,MichalDepta/FAKE,beeker/FAKE,Kazark/FAKE,rflechner/FAKE,MichalDepta/FAKE,rflechner/FAKE,MiloszKrajewski/FAKE,MiloszKrajewski/FAKE,MiloszKrajewski/FAKE,Kazark/FAKE,beeker/FAKE,rflechner/FAKE,MiloszKrajewski/FAKE,Kazark/FAKE,Kazark/FAKE