fix substreampartitionrouter state migration from child when it is empty

Author: maxi297

airbyte_cdk/sources/declarative/partition_routers/substream_partition_router.py

@@ -374,7 +374,7 @@ def _migrate_child_state_to_parent_state(self, stream_state: StreamState) -> Str
         # Ignore per-partition states or invalid formats.
         if isinstance(substream_state, (list, dict)) or len(substream_state_values) != 1:
             # If a global state is present under the key "state", use its first value.
-            if "state" in stream_state and isinstance(stream_state["state"], dict):
+            if "state" in stream_state and isinstance(stream_state["state"], dict) and stream_state["state"] != {}:
                 substream_state = list(stream_state["state"].values())[0]
             else:
                 return {}

unit_tests/sources/declarative/partition_routers/test_substream_partition_router.py

@@ -473,6 +473,19 @@ def test_substream_partition_router_invalid_parent_record_type():
             },
             {"parent_stream_cursor": "2023-05-27T00:00:00Z"},
         ),
+        # Case 7: Migrate child state to parent state but child state is empty
+        (
+                {
+                    "state": {},
+                    "states": [],
+                    "parent_state": {
+                        "posts": {}
+                    },
+                    "lookback_window": 1,
+                    "use_global_cursor": False
+                },
+                {},
+        ),
     ],
     ids=[
         "empty_initial_state",
@@ -481,6 +494,7 @@ def test_substream_partition_router_invalid_parent_record_type():
         "initial_state_no_parent_per_partition_state",
         "initial_state_with_parent_state",
         "initial_state_no_parent_global_state_declarative",
+        "initial_state_no_parent_and_no_child",
     ],
 )
 def test_set_initial_state(initial_state, expected_parent_state):