input: Write a function to detect a vowel in a given string. The function should return the first vowel found in the string. If no vowel is found, the function should return None. The input string may contain uppercase and lowercase letters.
  assert vowelCheck("Hello World")=='e'
  assert vowelCheck("Python")=='o'
  assert vowelCheck("XYZ")==None
  assert vowelCheck("")==None
  assert vowelCheck("AEIOU")=='A'
output: def vowelCheck(s):
      vowels = "aeiouAEIOU"
      for char in s:
          if char in vowels:
              return char
      return None