fix(utils.dicts): preserve empty dict leaf vals in deep merge (used i…

…n kedro conf merge)

src/mlopus/utils/dicts.py

@@ -94,12 +94,16 @@ def deep_merge(*dicts: dict):
     """Merge dicts at the level of leaf-values."""
     retval = {}
 
-    def _update(tgt: dict, src: dict, prefix_keys: List[str]):
+    def _update(tgt: dict, src: Mapping, prefix_keys: List[str]):
         for key, val in src.items():
-            if isinstance(val, dict):
-                _update(tgt, val, prefix_keys + [key])
+            _key = prefix_keys + [key]
+
+            if isinstance(val, Mapping) and (val or isinstance(get_nested(tgt, _key, None), Mapping)):
+                # Treat value as nested if it's a non-empty dict or if the target is already nested
+                _update(tgt, val, _key)
             else:
-                set_nested(tgt, prefix_keys + [key], deepcopy(val))
+                # Treat value as a leaf (scalar) otherwise
+                set_nested(tgt, _key, deepcopy(val))
 
     for _dict in dicts:
         _update(retval, _dict, prefix_keys=[])