input: Write a function to evaluate a numerical expression given as a string and return the result. If the input string is empty, return None.
  def eval_expr(expr):
     if expr == "":
         return None
     return eval(expr)
  
  assert eval_expr("2+3*4")==14
  assert eval_expr("10/2")==5.0
  assert eval_expr("7-3")==4
  assert eval_expr("")==None
output: def eval_expr(expr):
     if expr == "":
         return None
     return eval(expr)