When receiving an empty api response, only reset the measurements. (#2)

Currently all measurement keys are removed when receiving an empty
response. The object will not recover from this and will log
an exception when accessing `self.values.keys()` on a subsequent,
succesful, response.

Author: jvanderneutstulen

luftdaten/__init__.py

@@ -43,7 +43,8 @@ async def get_data(self):
             raise exceptions.LuftdatenConnectionError()
 
         if not self.data:
-            self.values = self.meta = None
+            for measurement in self.values.keys():
+                self.values[measurement] = None
             return
 
         try: