[variable] check if value is not none when casting

pyheatpump/models/variable.py

@@ -97,7 +97,8 @@ class Variable(RowClass):
                 AND var.last_update > :since""",
             params)
 
-            if 'address' in row.keys() and 'value' in row.keys()
+            if 'address' in row.keys() and 'value' in row.keys() and
+                row['value'] is not None
         }