Clear lastButtonClicked ref if it was removed from the DOM

[Telroshan]

This addresses #1749

• htmx uses an internal data to keep track of the last clicked button in a form
• it relies on the focusout event to clear that reference
• on Chrome, the focusout event gets called when the element is swapped and removed from the DOM
• however on Firefox for some reason, the event isn't fired at all, so the reference is never cleared, the internal state still has a reference to the removed button as the last clicked button, thus its values gets included, until you click out to focusout the form

This change adds a check right before using that lastButtonClicked property, making sure the referenced button is still in the DOM