Treat blank [formaction] like a blank [action]

When handling form submissions, a `<form>` element that declares a
`[action]` to be the empty string (i.e. `""`) is treated as if it were
targeting the current path:

> The `action` IDL attribute must reflect the content attribute of the
> same name, except that on getting, when the content attribute is
> missing or its value is the empty string, the element's node
> document's URL must be returned instead.
>
> [§ 4.10.18.6 Form Submission attributes][action]

Similarly, when handling form submissions with a submitter that declares
a `[formaction]` to be the empty string (i.e. `""`) is treated as if it
were targetting the current path.

> The `formAction` IDL attribute must reflect the formaction content
> attribute, except that on getting, when the content attribute is missing
> or its value is the empty string, the element's node document's URL must
>be returned instead.
>
> [§ 4.10.18.6 Form Submission attributes][formaction]

[action]: https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#dom-fs-action
[formaction]: https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#dom-fs-formaction

src/core/drive/form_submission.ts

@@ -74,7 +74,12 @@ export class FormSubmission {
 
   get action(): string {
     const formElementAction = typeof this.formElement.action === 'string' ? this.formElement.action : null
-    return this.submitter?.getAttribute("formaction") || this.formElement.getAttribute("action") || formElementAction || ""
+
+    if (this.submitter?.hasAttribute("formaction")) {
+      return this.submitter.getAttribute("formaction") || ""
+    } else {
+      return this.formElement.getAttribute("action") || formElementAction || ""
+    }
   }
 
   get body() {

src/tests/fixtures/form.html

@@ -93,6 +93,11 @@ <h1>Form</h1>
         <button type="submit" name="button">Submit</button>
       </form>
     </div>
+    <div id="blank-formaction">
+      <form action="/src/tests/fixtures/one.html">
+        <button formaction="">Submit</button>
+      </form>
+    </div>
     <hr>
     <div id="action-input">
       <form class="no-action">

src/tests/functional/form_submission_tests.ts

@@ -283,6 +283,14 @@ export class FormSubmissionTests extends TurboDriveTestCase {
     this.assert.deepEqual(await this.getAllSearchParams("button"), [])
   }
 
+  async "test submitter with blank formaction submits to the current page"() {
+    await this.clickSelector("#blank-formaction button")
+    await this.nextBody
+
+    this.assert.ok(await this.hasSelector("#blank-formaction"), "overrides form[action] navigation")
+    this.assert.equal(await this.pathname, "/src/tests/fixtures/form.html")
+  }
+
   async "test input named action with no action attribute"() {
     await this.clickSelector("#action-input form.no-action [type=submit]")
     await this.nextBody