input: Write a function that takes a string containing a numerical expression and returns its evaluated result. The function should use Python's `eval` function to compute the result of the expression. If the input string is empty, the function should return `None`.
  
  def eval_expr(expr):
      if not expr:
          return None
      try:
          return eval(expr)
      except:
          return None
  
  assert eval_expr("2+2") == 4
  assert eval_expr("3*5") == 15
  assert eval_expr("10/2") == 5.0
  assert eval_expr("10/0") == None
  assert eval_expr("") == None
output: def eval_expr(expr):
      if not expr:
          return None
      try:
          return eval(expr)
      except:
          return None