Avoid setting reCAPTCHA token on failed execute

[aduth]

🛠 Summary of changes

Updates the implementation of the reCAPTCHA submit button to avoid setting a token value if reCAPTCHA fails to execute.

Previously, if reCAPTCHA failed to execute (throws an error), the value would be undefined, which would be stringified to the form input as the literal string "undefined". By not setting the token value, we can get more accurate error details on the backend for a "blank" form error reason (source).

📜 Testing Plan

yarn app/javascript/packages/captcha-submit-button/captcha-submit-button-element.spec.ts

Previously, this would fail with:

      AssertionError: expected { recaptcha_token: 'undefined' } to deeply equal { recaptcha_token: '' }
      + expected - actual

       {
      -  "recaptcha_token": "undefined"
      +  "recaptcha_token": ""
       }

Verify token is sent empty for failed reCAPTCHA execute:

1. Repeat testing plan from Add error handling for failed reCAPTCHA execute #11449, with browser developer tools "Network" tab open, and "Preserve log" checked
2. After clicking "Sign in", observe click on the POST request to the sign-in route
3. Click "Payload" tab
4. Observe that recaptcha_token is empty, rather than "undefined"

[aduth]

If this TypeScript type were assigned previously, it'd also have been flagged as an issue:

app/javascript/packages/captcha-submit-button/captcha-submit-button-element.ts:65:7 - error TS2322: Type 'string | undefined' is not assignable to type 'string'.
  Type 'undefined' is not assignable to type 'string'.

65       this.tokenInput.value = token;
         ~~~~~~~~~~~~~~~~~~~~~