Stream default to Stream::Null() when no default in function prototype

this corrects bug opencv#16592 where a Stream is created at
each GpuMat::load(arr,stream) call

a correct solution would have been to add a default to GpuMat::load
but due to circular dependence between Stream and GpuMat, this is not possible

modules/python/src2/gen2.py

@@ -211,6 +211,7 @@ class FormatStrings:
     "double": ArgTypeInfo("double", FormatStrings.double, "0", True),
     "c_string": ArgTypeInfo("char*", FormatStrings.string, '(char*)""'),
     "string": ArgTypeInfo("std::string", FormatStrings.object, None, True),
+    "Stream": ArgTypeInfo("Stream", FormatStrings.object, 'Stream::Null()', True),
 }