input: Write a function to detect the first vowel in a given string. If a vowel is found, return that vowel; if no vowel is found, return None. 
  assert vowelCheck("Hello World")=="e"
  assert vowelCheck("H3ll0 W0rld")==None
  assert vowelCheck("pyth0n")==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