input: Write a function to evaluate a given string containing a numerical expression and return the result. The function should use the eval() function to evaluate the expression. If the input is an empty string, return None.
  
  def eval_expr(expr):
     if not expr:
        return None
     return eval(expr)
  
  assert eval_expr('2+3*4')==14
  assert eval_expr('10/2')==5.0
  assert eval_expr('5**2')==25
  assert eval_expr('3/0')==None # Division by zero should be handled
  assert eval_expr('')==None # Empty string should return None
output: def eval_expr(expr):
     if not expr:
        return None
     try:
        return eval(expr)
     except:
        return None