Fix bug where SSR is dispatched twice when errors exist

[reinink]

This PR fixes a bug with the Inertia Blade directives that cause the SSR node server to be called twice in the event of a JavaScript related failure.

This happens because the \Inertia\Ssr\Gateway returns either an \Inertia\Ssr\Response (on success), or null on failure (a JavaScript error), which is saved in the $__inertiaSsr variable, and then we check to see if that variable isset to prevent calling the SSR server twice. Problem is, in PHP calling isset() on a variable that's null returns false, even if that variable has been set.

This PR fixes this by introducing a new $__inertiaSsrDispatched variable to keep track of whether the SSR server has been dispatched, regardless of the response.