fix(signals): simplify x-html null-guard to '?? ""' (#1767)

The #1768 null-guard rewrote 'el.innerHTML = evalAttrExpr(value)' into a
'var __html = ...' form, which broke the scope-lifecycle 'should handle @html'
assertion that checks the runtime still contains 'el.innerHTML = evalAttrExpr'.
Use the runtime's existing '?? '' idiom (cf. el.value at signals.ts) so the
null/undefined coercion stays AND the asserted shape is preserved. Full suite
green (7796 pass, 0 fail).

Author: glennmichael123

packages/stx/src/signals.ts

@@ -1861,8 +1861,7 @@ else if (name === '@html' || name === ':html' || name === 'x-html') {
         effect(() => {
           // Coerce null/undefined to '' so a missing/unresolved binding renders
           // nothing rather than the literal string "undefined" (#1767).
-          var __html = evalAttrExpr(value);
-          el.innerHTML = (__html == null) ? '' : __html;
+          el.innerHTML = evalAttrExpr(value) ?? '';
         });
         el.removeAttribute(name);
       }