input: function
  Write a function to evaluate a given numerical expression string and return the result. If the input string is empty or invalid, return None. The function should handle basic arithmetic operations such as addition, subtraction, multiplication, and division.
  assert eval_expr("1+2*3")==7
  assert eval_expr("(4+5)/2")==4.5
  assert eval_expr("6-3+2")==5
  assert eval_expr("5*0")==0
  assert eval_expr("")==None
  
output:     
      try:
          return eval(expr)
      except:
          return None
  