Check for empty Mat in compare, operator= and RNG::fill, fixed related tests

modules/core/src/rand.cpp

@@ -511,6 +511,8 @@ static RandnScaleFunc randnScaleTab[] =
 void RNG::fill( InputOutputArray _mat, int disttype,
                 InputArray _param1arg, InputArray _param2arg, bool saturateRange )
 {
+    if (_mat.empty())
+        return;
     Mat mat = _mat.getMat(), _param1 = _param1arg.getMat(), _param2 = _param2arg.getMat();
     int depth = mat.depth(), cn = mat.channels();
     AutoBuffer<double> _parambuf;