fix: es8 Field.serialize with skip_empty

Author: aaxelb

osf/metrics/es8_metrics.py

@@ -37,7 +37,7 @@ def deserialize(self, data):
         except ValueError:
             raise ValueError(f'unsure how to deserialize "{data}" (of type {type(data)}) to YearMonth')
 
-    def serialize(self, data):
+    def serialize(self, data, skip_empty=True):
         if isinstance(data, str):
             return data
         elif isinstance(data, YearMonth):