input: Write a function to detect the first vowel in a given string. The function should return the first vowel found or None if no vowels are present.
  assert vowelCheck("Hello World")=="e"
  assert vowelCheck("Python")=="o"
  assert vowelCheck("TV")==None
  assert vowelCheck("aeiou")=="a"
  assert vowelCheck("")==None
output: def vowelCheck(s):
      vowels = "aeiouAEIOU"
      for char in s:
          if char in vowels:
              return char
      return None