feat: ensure annotation keys always present in serialized output

Modify AnnotationsMixin.serialize_annotations() to always include
'text-annotations' and 'range-annotations' keys in the returned dict,
even when the lists are empty. Previously, these keys were only added
when annotations existed.

This change ensures consistent API response structure and prevents
potential KeyError issues when consumers expect these keys to always
be present.

Author: palewire

datawrapper/charts/models/mixins.py

@@ -299,20 +299,25 @@ def _serialize_annotations(
 
         Returns:
             dict: Annotations in API format with keys:
-                - text-annotations: List of text annotation dicts
-                - range-annotations: List of range annotation dicts
+                - text-annotations: List of text annotation dicts (always present, may be empty)
+                - range-annotations: List of range annotation dicts (always present, may be empty)
         """
         result = {}
 
-        if self.text_annotations:
-            result["text-annotations"] = ModelListSerializer.serialize(
-                self.text_annotations, text_annotation_class
-            )
+        # Always include annotation keys, even when empty
+        result["text-annotations"] = (
+            ModelListSerializer.serialize(self.text_annotations, text_annotation_class)
+            if self.text_annotations
+            else []
+        )
 
-        if self.range_annotations:
-            result["range-annotations"] = ModelListSerializer.serialize(
+        result["range-annotations"] = (
+            ModelListSerializer.serialize(
                 self.range_annotations, range_annotation_class
             )
+            if self.range_annotations
+            else []
+        )
 
         return result