input: Write a function to detect the first vowel in a given string. If a vowel is found, return it. If no vowel is found, return None.
  assert vowelCheck("Hello World")=='e'
  assert vowelCheck("pythn")==None
  assert vowelCheck("AEIOU")=='A'
  assert vowelCheck("")==None
  assert vowelCheck("bcdfgh")==None
output: def vowelCheck(s):
      vowels = "aeiouAEIOU"
      for char in s:
          if char in vowels:
              return char
      return None