Fix annotator-params default to empty dict instead of None (#488)

Co-authored-by: Tatjana Damnjanović <tatjana@solvebio.com>

Author: damnjanovictanja

solvebio/query.py

@@ -818,8 +818,8 @@ def export(self, format='json', follow=True, limit=None, **kwargs):
         # Prefer explicitly passed-in values before query values.
         target_fields = kwargs.pop('target_fields', None) or \
             params.pop('target_fields', None)
-        annotator_params = kwargs.pop('annotator_params', None) or \
-            params.pop('annotator_params', None)
+        annotator_params = kwargs.pop('annotator_params', {}) or \
+            params.pop('annotator_params', {})
 
         export = DatasetExport.create(
             dataset_id=self._dataset_id,
@@ -872,8 +872,8 @@ def migrate(self, target, follow=True, **kwargs):
         # Prefer explicitly passed-in values before query values.
         target_fields = kwargs.pop('target_fields', None) or \
             params.pop('target_fields', None)
-        annotator_params = kwargs.pop('annotator_params', None) or \
-            params.pop('annotator_params', None)
+        annotator_params = kwargs.pop('annotator_params', {}) or \
+            params.pop('annotator_params', {})
 
         migration_resp = DatasetMigration.create(
             source_id=self._dataset_id,