WriteMultipleRegistersRequest: Handle values=None case

Currently, if None is passed in explicitly, or if values is not given,
the 'values' object is correctly identified as *not* having an __iter__
attribute, but is incorrectly identified as being a valid register value.
This breaks testInvalidWriteMultipleRegistersRequest.

Solution: if we see None, replace this with [] and skip the check for
__iter__.

pymodbus/register_write_message.py

@@ -132,9 +132,11 @@ def __init__(self, address=None, values=None, **kwargs):
         '''
         ModbusRequest.__init__(self, **kwargs)
         self.address = address
-        self.values = values or []
-        if not hasattr(values, '__iter__'):
-            self.values = [values]
+        if values is None:
+            values = []
+        elif not hasattr(values, '__iter__'):
+            values = [values]
+        self.values = values
         self.count = len(self.values)
         self.byte_count = self.count * 2