Critical fix: MWT detection was incorrectly including empty nodes

Issue: Empty nodes (8, '.', 1) were being added to BOTH mwts and empty_nodes:
- As MWT: "8-1" (wrong - invalid ID)
- As empty_node: "8.1" (correct)

Root cause: MWT detection checked isinstance(token['id'], tuple) without
checking the middle element. Both MWTs and empty nodes are tuples:
- MWT: (1, '-', 2) for "1-2"
- Empty node: (8, '.', 1) for "8.1"

Fix: Add token['id'][1] == '-' check to MWT detection to exclude empty nodes

This caused 75 validation errors in sentences with empty nodes.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

tools/04_generate_parquet.py

@@ -90,9 +90,11 @@ def extract_examples_from_conllu(filepath: str) -> List[Dict[str, Any]]:
                 sent_idx = str(idx)
 
             # Extract Multi-Word Tokens (MWTs) - tokens with tuple IDs like (1, '-', 2)
+            # Note: Exclude empty nodes which have '.' as middle element: (22, '.', 1)
             mwts = []
             for token in sent:
-                if isinstance(token["id"], tuple):  # MWT line (e.g., "1-2")
+                if isinstance(token["id"], tuple) and len(token["id"]) == 3 and token["id"][1] == '-':
+                    # MWT line (e.g., (1, '-', 2) for "1-2")
                     mwts.append({
                         "id": f"{token['id'][0]}-{token['id'][2]}",
                         "form": token["form"],

tools/templates/universal_dependencies.tmpl

@@ -194,9 +194,11 @@ class UniversalDependencies(datasets.GeneratorBasedBuilder):
                         idx = id
 
                     # Extract Multi-Word Tokens (MWTs) - tokens with tuple IDs like (1, '-', 2)
+                    # Note: Exclude empty nodes which have '.' as middle element: (22, '.', 1)
                     mwts = []
                     for token in sent:
-                        if isinstance(token["id"], tuple):  # MWT line (e.g., "1-2")
+                        if isinstance(token["id"], tuple) and len(token["id"]) == 3 and token["id"][1] == '-':
+                            # MWT line (e.g., (1, '-', 2) for "1-2")
                             mwts.append({
                                 "id": f"{token['id'][0]}-{token['id'][2]}",
                                 "form": token["form"],