input: Write a function to evaluate a numerical expression given as a string and return the result. Handle possible errors during evaluation by returning None if the expression is invalid or otherwise cannot be evaluated. If the input string is empty, return None.
  assert eval_expr("10 + 3 * 4")==22
  assert eval_expr("(7 - 2) / 5")==1.0
  assert eval_expr("9 ** 2")==81
  assert eval_expr("abc")==None
  assert eval_expr("")==None
output: def eval_expr(expr):
      if not expr:
          return None
      try:
          return eval(expr)
      except:
          return None