Bugfix: move construct an empty fast_io::list

[SekaiArendelle]

• before this patch, the list_move.cc#L10 will crash

Following information comes from GDB to prove why the move constructor before this patch is wrong:

gef➤  p result
$6 = {
  static alloc_with_status = 0x0,
  allochdl = {
    value = {
      static has_status = 0x0
    }
  },
  imp = {
    prev = 0x7fffffffd528,
    next = 0x7fffffffd528
  }
}
gef➤  p &result
$7 = (pltxt2htm::details::MdListAst *) 0x7fffffffd528


gef➤  p call_stack.top().result
$8 = {
  static alloc_with_status = 0x0,
  allochdl = {
    value = {
      static has_status = 0x0
    }
  },
  imp = {
    prev = 0x7fffffffd528,
    next = 0x7fffffffd528
  }
}
gef➤  p &call_stack.top().result
$9 = (pltxt2htm::details::MdListAst *) 0x55555556f548 // Mistake here

[trcrsired]

use is_empty() instead. thanks

[SekaiArendelle]

@trcrsired Done