Fix cjk handling in SerializeAddon

[mmis1000]

Use cell.getWidth() to get correct cell width instead of assume 1 width of every empty cell.

xterm.js/addons/xterm-addon-serialize/src/SerializeAddon.ts

Lines 140 to 142 in 106ea62

	if (cell.getChars() === '') {
	this._nullCellCount++;
	} else if (this._nullCellCount > 0) {

Fix #3066

[jerch]

Nice find, LGTM 👍

On the background of this: We actually have 2 types of null cells - the default one has a width of 1, which still accounts towards cursor offsets and has a default output representation of one SP. The second one appears after full width chars with a width of 0, which does not account anywhere and has no own output repr. Thus it is important to grab the true cell width for nullish cells to correctly calculate cursor offsets and/or filling spaces afterwards.

[Tyriar]

Thanks 👍