abort find_alignment on empty input

whisper/timing.py

@@ -170,6 +170,9 @@ def find_alignment(
     medfilt_width: int = 7,
     qk_scale: float = 1.0,
 ) -> List[WordTiming]:
+    if len(text_tokens) == 0:
+        return []
+
     tokens = torch.tensor(
         [
             *tokenizer.sot_sequence,