patch 8.1.0164: luaeval('vim.buffer().name') returns an error

Problem:    luaeval('vim.buffer().name') returns an error.
Solution:   Return an empty string. (Dominique Pelle, closes #3167)

src/if_lua.c

@@ -1123,9 +1123,11 @@ luaV_buffer_index(lua_State *L)
    {
	const char *s = lua_tostring(L, 2);
	if (strncmp(s, "name", 4) == 0)
	    lua_pushstring(L, (char *) b->b_sfname);
	    lua_pushstring(L, (b->b_sfname == NULL)
					? "" : (char *) b->b_sfname);
	else if (strncmp(s, "fname", 5) == 0)
	    lua_pushstring(L, (char *) b->b_ffname);
	    lua_pushstring(L, (b->b_ffname == NULL)
					? "" : (char *) b->b_ffname);
	else if (strncmp(s, "number", 6) == 0)
	    lua_pushinteger(L, b->b_fnum);
	/* methods */

src/testdir/test_lua.vim

@@ -198,11 +198,8 @@ endfunc
" Test vim.buffer().name and vim.buffer().fname
func Test_buffer_name()
  new
  " FIXME: for an unnamed buffer, I would expect
  " vim.buffer().name to give an empty string, but
  " it returns 0. Is it a bug?
  " so this assert_equal is commented out.
  " call assert_equal('', luaeval('vim.buffer().name'))
  call assert_equal('', luaeval('vim.buffer().name'))
  call assert_equal('', luaeval('vim.buffer().fname'))
  bwipe!

  new Xfoo

src/version.c

@@ -789,6 +789,8 @@ static char *(features[]) =

static int included_patches[] =
{   /* Add new patch number below this line */
/**/
    164,
/**/
    163,
/**/