Make payload optional again for create tag API

Before galaxyproject#17064 it was
possible to create a tag by simply making a POST request like
`/api/histories/911dde3ddb677bcd/tags/tag1` , this restores the
previous behaviour.

Fix BioBlend test failure:

https://github.com/galaxyproject/bioblend/actions/runs/7094150509/job/19308900670#step:10:3029

lib/galaxy/schema/item_tags.py

@@ -42,7 +42,7 @@ class ItemTagsListResponse(Model):
 class ItemTagsCreatePayload(Model):
     """Payload schema for creating an item tag."""
 
-    value: str = Field(
-        Required,
+    value: Optional[str] = Field(
+        None,
         title="value of the item tag",
     )

lib/galaxy/webapps/galaxy/api/item_tags.py

@@ -72,8 +72,10 @@ def create(
                 trans: ProvidesAppContext = DependsOnTrans,
                 item_id: DecodedDatabaseIdField = Path(..., title="Item ID", alias=tagged_item_id),
                 tag_name: str = Path(..., title="Tag Name"),
-                payload: ItemTagsCreatePayload = Body(...),
+                payload: ItemTagsCreatePayload = Body(None),
             ) -> ItemTagsResponse:
+                if payload is None:
+                    payload = ItemTagsCreatePayload()
                 return self.manager.create(trans, tagged_item_class, item_id, tag_name, payload)
 
             @router.put(

lib/galaxy_test/api/test_item_tags.py

@@ -1,3 +1,8 @@
+from typing import (
+    Any,
+    Dict,
+)
+
 from galaxy_test.base.populators import (
     DatasetCollectionPopulator,
     DatasetPopulator,
@@ -125,7 +130,7 @@ def _test_delete_tag(self, prefix):
 
     def _create_valid_tag(self, prefix: str):
         url = f"{prefix}/tags/awesometagname"
-        tag_data = dict(value="awesometagvalue")
+        tag_data: Dict[str, Any] = {}  # Can also be dict(value="awesometagvalue")
         response = self._post(url, data=tag_data, json=True)
         return response