Fix handling of optional params in blockCopy()

Commit 2b4bd07e0a (Add check for params, nparams being a dictionary)
changed the way the optional params argument is treated. If
libvirt.virDomain.blockCopy() is called without specifying params,
params is None, and the call will fail with:

    TypeError: block params must be a dictionary

This is wrong as params is defined as kwarg, breaking existing libvirt
users like oVirt. Add a check for Py_None, so we accept either a dict or
None and fail with TypeError with anything else.

Resolves: https://bugzilla.redhat.com/1687114

Signed-off-by: Nir Soffer <nsoffer@redhat.com>

libvirt-override.c

@@ -8837,7 +8837,7 @@ libvirt_virDomainBlockCopy(PyObject *self ATTRIBUTE_UNUSED,
                                    VIR_N_ELEMENTS(virPyDomainBlockCopyParams)) < 0) {
             return NULL;
         }
-    } else {
+    } else if (pyobj_dict != Py_None) {
         PyErr_Format(PyExc_TypeError, "block params must be a dictionary");
         return NULL;
     }