fix: guard nighit, check_sara, check_marttra against empty input

nighit() crashed with IndexError when w2 had no consonants.
check_sara("") and check_marttra("") crashed accessing word[-1].
Add input validation with clear error messages.

Author: phoneee

pythainlp/khavee/core.py

@@ -54,6 +54,9 @@ def check_sara(self, word: str) -> str:
         sara = []
         countoa = 0
 
+        if not word:
+            return ""
+
         # In case of การันย์
         if "์" in word[-1]:
             word = word[:-2]
@@ -253,6 +256,9 @@ def check_marttra(self, word: str) -> str:
         word = self.handle_karun_sound_silence(word)
         word = remove_tonemark(word)
 
+        if not word:
+            return ""
+
         # Check for ำ at the end (represents "am" sound, ends with m)
         if word[-1] == "ำ":
             return "กม"

pythainlp/morpheme/word_formation.py

@@ -38,7 +38,12 @@ def nighit(w1: str, w2: str) -> str:
     newword = []
     newword.append(list_w1[0])
     newword.append("ั")
-    consonant_start = [i for i in list_w2 if i in set(thai_consonants)][0]
+    consonants_in_w2 = [i for i in list_w2 if i in set(thai_consonants)]
+    if not consonants_in_w2:
+        raise ValueError(
+            f"w2 '{w2}' contains no Thai consonants."
+        )
+    consonant_start = consonants_in_w2[0]
     if consonant_start in ["ก", "ช", "ค", "ข", "ง"]:
         newword.append("ง")
     elif consonant_start in ["จ", "ฉ", "ช", "ฌ"]: