perf: improve common element attribute getters

In main, there's a `getId`, and `getClassName` (etc...) getter on Element. But
these all `orelse ""`, because that's what the WebAPI wants. Internally though,
most code want the optional. The result is that _many_ places do:

```zig
el.getAttributeSafe(comptime .wrap("id"))
```

instead of:

```zig
el.getId()
```

This is a bit tedious AND, it means that when we improve `Element.getId` (1) no
internal caller benefits from it. This commit makes the element getters return
the optional (`?[]const u8`) and updates every callsite to use the new getter.
The `orelse ""` needed by the WebAPI is moved to the JsApi bridge.

(1) https://github.com/lightpanda-io/browser/pull/3457