Add guard clause to return valid empty form data

[julianrubisch]

Bug fix

Description

After a lot of headache @marcoroth and me found out that new FormData(null) throws an error in Chrome, but does not in safari.

The closest('form') call in stimulate would return null in case no form is present up the DOM hierarchy, blowing up serializeForm, but just on Chrome 😂

Checklist

• My code follows the style guidelines of this project
• Checks (StandardRB & Prettier-Standard) are passing
• This is not a documentation update

[jonathan-s]

Throws an error in firefox as well. So fixes it for firefox too.

[marcoroth]

Yeah, also throws an error in Opera. Seems like Safari is just a little bit more forgiving 🙈

[marcoroth]

@julianrubisch maybe we can add another test case for this in https://github.com/hopsoft/stimulus_reflex/blob/888c2635e4cc65efaca1d92d3b2d6949c3d91758/javascript/test/utils.serializeForm.test.js#L5

[jonathan-s]

I'd be happy to volunteer some help to set up cypress. It could have helped in a situation like this. I'd need some help on the rails part to get started though.

[julianrubisch]

@ParamagicDev is our man for Cypress 😎