fix: exclude optional collection values

If we're not using response models, we can't automagically exclude none values.

stac_fastapi/sqlalchemy/stac_fastapi/sqlalchemy/serializers.py

@@ -146,22 +146,28 @@ def db_to_stac(cls, db_model: database.Collection, base_url: str) -> TypedDict:
         if db_links:
             collection_links += resolve_links(db_links, base_url)
 
-        stac_extensions = db_model.stac_extensions or []
-
-        return stac_types.Collection(
+        collection = stac_types.Collection(
             type="Collection",
             id=db_model.id,
-            stac_extensions=stac_extensions,
             stac_version=db_model.stac_version,
-            title=db_model.title,
             description=db_model.description,
-            keywords=db_model.keywords,
             license=db_model.license,
-            providers=db_model.providers,
-            summaries=db_model.summaries,
             extent=db_model.extent,
             links=collection_links,
         )
+        # We need to manually include optional values to ensure they are
+        # excluded if we're not using response models.
+        if db_model.stac_extensions:
+            collection.stac_extensions = db_model.stac_extensions
+        if db_model.title:
+            collection.title = db_model.title
+        if db_model.keywords:
+            collection.keywords = db_model.keywords
+        if db_model.providers:
+            collection.providers = db_model.providers
+        if db_model.summaries:
+            collection.summaries = db_model.summaries
+        return collection
 
     @classmethod
     def stac_to_db(