Mock ParameterFilter fix

ParameterFilter blocks are no longer forced to return a boolean value.  Since any value can be implicitly cast to Boolean anyway, this wasn't adding any value.

Also, the initial call to Test-ParameterFilter which would enforce this condition could sometimes wind up throwing errors due to null reference exceptions, in cases where the filter assumes that a mandatory parameter must not be null.

Partial fix for #218 .  (The example noted still needs to be updated, because $Path is not a mandatory parameter and could be null anyway.)