fix: correct functions based on failing tests

Author: ecarrenolozano

ontograph/utils.py

@@ -63,9 +63,14 @@ def _create_reverse_mapping(
     Returns:
         A dictionary where keys are IDs from source columns and values are from the target_column.
     """
-    if dataframe.empty or target_column not in dataframe.columns:
+    if dataframe.empty:
         return set(), {}
 
+    if target_column not in dataframe.columns:
+        raise KeyError(
+            f"Target column '{target_column}' not found in DataFrame."
+        )
+
     # Ensure we only work with rows that have a target ID
     df_filtered = dataframe.dropna(subset=[target_column])