input: Write a function to evaluate a given string containing a numerical expression and return the result. If the input string is empty or contains non-numerical characters, return None.
  assert eval_expr("2+2")==4
  assert eval_expr("10*5-3")==47
  assert eval_expr("3/2")==1.5
  assert eval_expr("abc")==None
  assert eval_expr("")==None
output: def eval_expr(s):
      try:
          return eval(s)
      except:
          return None