forms: skip FormData entry for <select> with no selectedness candidate

Per the HTML Living Standard "constructing the form data set" algorithm,
a <select> element appends one entry per option whose selectedness is
true. A non-multiple <select> derives its selectedness from an
explicitly-selected option, falling back to the first non-disabled
option in tree order. With zero options (or only disabled options), no
option is selected, so no entry should be appended.

The singular-select fast path in collectForm previously broke from the
block with select.getValue(frame) regardless of whether any option
existed, causing FormData to emit a phantom (name, "") entry for an
empty <select>. Guard the fast path on the existence of at least one
non-disabled <option> child and skip the entry otherwise.

Closes #2262

src/browser/tests/net/form_data.html

@@ -813,6 +813,72 @@
 }
 </script>
 
+<script id=selectWithoutOptions>
+{
+  // Per HTML spec (constructing form data set, step 5.5), a <select> only
+  // appends entries for options whose selectedness is true. With zero options,
+  // nothing is selected and no entry is appended.
+  const form = document.createElement('form');
+
+  const sel = document.createElement('select');
+  sel.name = 'choice';
+  form.appendChild(sel);
+
+  const sibling = document.createElement('input');
+  sibling.name = 'sibling';
+  sibling.value = 'kept';
+  form.appendChild(sibling);
+
+  const fd = new FormData(form);
+  testing.expectEqual(false, fd.has('choice'));
+  testing.expectEqual(null, fd.get('choice'));
+  testing.expectEqual([], fd.getAll('choice'));
+  testing.expectEqual('kept', fd.get('sibling'));
+  testing.expectEqual([['sibling', 'kept']], Array.from(fd.entries()));
+}
+</script>
+
+<script id=selectMultipleWithoutOptions>
+{
+  // Same rule for <select multiple> — with zero options, no entry.
+  const form = document.createElement('form');
+
+  const sel = document.createElement('select');
+  sel.name = 'tags';
+  sel.multiple = true;
+  form.appendChild(sel);
+
+  const fd = new FormData(form);
+  testing.expectEqual(false, fd.has('tags'));
+  testing.expectEqual([], fd.getAll('tags'));
+  testing.expectEqual([], Array.from(fd.entries()));
+}
+</script>
+
+<script id=selectAllOptionsDisabled>
+{
+  // A single-select with only disabled options has no auto-selected option,
+  // so no entry is appended.
+  const form = document.createElement('form');
+
+  const sel = document.createElement('select');
+  sel.name = 'mode';
+
+  const opt = document.createElement('option');
+  opt.value = 'off';
+  opt.disabled = true;
+  opt.textContent = 'off';
+  sel.appendChild(opt);
+
+  form.appendChild(sel);
+
+  const fd = new FormData(form);
+  testing.expectEqual(false, fd.has('mode'));
+  testing.expectEqual(null, fd.get('mode'));
+  testing.expectEqual([], fd.getAll('mode'));
+}
+</script>
+
 <script id=formDataEventModifyFormData>
 {
   // Listeners can modify formData during the event

src/browser/webapi/net/FormData.zig

@@ -196,6 +196,17 @@ fn collectForm(arena: Allocator, form_: ?*Form, submitter_: ?*Element, frame: *F
 
             if (element.is(Form.Select)) |select| {
                 if (select.getMultiple() == false) {
+                    // Per the HTML spec, a single-select's selectedness comes
+                    // from an explicitly-selected option, falling back to the
+                    // first non-disabled option in tree order. With no options
+                    // (or only disabled ones), nothing is selected and no
+                    // entry is appended.
+                    var children = select.asNode().childrenIterator();
+                    const has_candidate = while (children.next()) |child| {
+                        const option = child.is(Form.Select.Option) orelse continue;
+                        if (!option.getDisabled()) break true;
+                    } else false;
+                    if (!has_candidate) continue;
                     break :blk select.getValue(frame);
                 }