Fix compilation error with C++23

[Nukoooo]

Before fix it gives this error:

amtl/am-string.h:184:16: error: conversion function from 'std::nullptr_t' to 'std::string' (aka 'basic_string<char>') invokes a deleted function
  184 |         return nullptr;

Returning an empty string fixes the problem and works as intended according to this:

There are several possible ways to fix this code; you could pass an empty string with "" or std::string{} or even {} (depending on whether open_choices() is overloaded).

from mamedev/mame#8265

[dvander]

Thanks!