[9.x] Handle undefined array key error

[ksassnowski]

Fixes #42605

Oh boy, the gift that keeps on giving. Looks like there's yet another edge case.

Summary

This PR fixes a regression when trying passing a model to the route helper for a route without route parameters.

$model = new User(['id' => 1]);

Route::get('/foo')->name('foo');

route('foo', $model); // Used to return `/foo?1`, now throws an exception

While I think this behavior is pretty weird, I guess it shouldn't break between minor versions.

[driesvints]

At least we're adding tests for all of this 👍