update tone ordering key

Author: WTFPUn

pythainlp/util/collate.py

@@ -15,7 +15,9 @@
 def _thkey(word: str) -> str:
     cv = _RE_TONE.sub("", word)  # remove tone
     cv = _RE_LV_C.sub("\\2\\1", cv)  # switch lead vowel
-    tone = _RE_TONE.sub(" ", word)  # just tone
+    
+    tone_match = _RE_TONE.search(word)
+    tone = tone_match.group() if tone_match else "" 
     return cv + tone