bug: check_sara, check_marttra, nighit crash on empty or vowel-only input

Description

Three public functions crash with IndexError when given empty strings or inputs lacking expected characters:

1. KhaveeVerifier.check_sara("") → IndexError at line 58 — word[-1] on empty string
2. KhaveeVerifier.check_marttra("") → IndexError at line 257 — word[-1] on empty string
3. nighit("สํ", "าี") → IndexError at line 41 — [...][0] on empty list when w2 has no Thai consonants

Expected results

kv.check_sara("")       # → "" or raise ValueError
kv.check_marttra("")    # → "" or raise ValueError
nighit("สํ", "าี")     # → raise ValueError

Current results

kv.check_sara("")       # IndexError: string index out of range
kv.check_marttra("")    # IndexError: string index out of range
nighit("สํ", "าี")     # IndexError: list index out of range

Steps to reproduce

from pythainlp.khavee import KhaveeVerifier
kv = KhaveeVerifier()
kv.check_sara("")        # crash
kv.check_marttra("")     # crash

from pythainlp.morpheme import nighit
nighit("สํ", "าี")      # crash

PyThaiNLP version

5.3.3

Python version

3.13

Operating system and version

macOS

Possible solution

• check_sara/check_marttra: Add if not word: return "" at function start.
• nighit: Check consonant list non-empty before [0], raise ValueError with descriptive message.

Files

• pythainlp/khavee/core.py (lines 58, 257)
• pythainlp/morpheme/word_formation.py (line 41)

Start with pythainlp/khavee/core.py at check_sara and check_marttra, then inspect pythainlp/morpheme/word_formation.py at nighit. Reproduce the empty and vowel-only inputs from the issue and find the existing tests for these functions. Add regression coverage and ensure the functions no longer produce IndexError, using the intended empty-result or ValueError behavior.