input: Write a function that takes a string containing a numerical expression and returns the result. The function should use the Python built-in `eval()` function to evaluate the expression.
  
  ### Function Implementation:
  def eval_expr(expr):
      return eval(expr)
  
  ### Test Cases:
  assert eval_expr('2 + 3 * 4')==14
  assert eval_expr('(2 + 3) * 4')==20
  assert eval_expr('10 / 2 + 3')==8.0
  assert eval_expr('2 ** 3 - 4')==4
  assert eval_expr('20 % 3')==2
  assert eval_expr('')==None
output: def eval_expr(expr):
      if expr == '':
          return None
      return eval(expr)