[monodoc] Ensure we don't try to use <Parameters> when it might be null. Fixes Gendarme doc generation.

As is, the assumption is made that op_Foobar method signatures which are not unary, such as binary or comparison operators, will have valid parameters defined - these are used to generate a friendly method signature. In some broken cases (such as Gendarme), this is not guaranteed, so there may be no parameters, causing a NullReferenceException to be thrown.