input: Write a function to detect the vowel in a given string. The function should return the first vowel it encounters. If there are no vowels in the string, the function should return None.
  assert vowelCheck("Hello World")=='e'
  assert vowelCheck("Python")=='o'
  assert vowelCheck("Rhythm")==None
output: def vowelCheck(s):
      vowels = "aeiouAEIOU"
      for char in s:
          if char in vowels:
              return char
      return None