input: Write a function to evaluate a numerical expression string and return its result. The function should use Python's built-in eval() function to compute the result of the given expression string. If the input string is empty or invalid, the function should return None.
  
  assert eval_expr("10 + 2 - 3 * 4") == 6
  assert eval_expr("5 * (2 + 3)") == 25
  assert eval_expr("2 ** 3") == 8
  assert eval_expr("") == None
  assert eval_expr("7 / 0") == None
output: def eval_expr(expr):
      try:
          if not expr:
              return None
          return eval(expr)
      except:
          return None