[8.x] Fix appendable attributes in Blade components

[panda-madness]

Currently, attempting to render a component which prepends a non-class attribute in it's template without actually overriding it in the calling template yields an Exception.

For example, given the following component called test-component:

<div {{ $attributes->merge(['data-controller' => $attributes->prepends('profile-controller')]) }}>
    {{ $slot }}
</div>

Rendering this component like so: <x-test-component data-controller="some-controller" /> works as expected.

Rendering this component like so: <x-test-component /> yields the following error:

This PR fixes this behavior. <x-test-component /> will now properly render as

<div data-controller="profile-controller"></div>

See #35128 for more details.

[driesvints]

@panda-madness it's also best that you provide a thorough explanation in your PR description (see our PR templates) so Taylor knows what this is about and why it's needed. Don't just link to an issue.

[panda-madness]

@driesvints I've updated the PR description.

I've also modified the test to include this use case, but I'm not quite sure why an unrelated test started failing...

[driesvints]

@panda-madness your whitespace change from here seems to be the culprit: 8b2e529#diff-9e28a6d9f9f352efc3960eab4ff2d081a25459d765d3f1ac2c9735dd4356dd50L45

Think I got it now.

[panda-madness]

This is a little nitpicky, but should we change the AppendableAttributeValue class name to PrependableAttributeValue so it maches the method name better?

[driesvints]

@panda-madness probably not worth the breaking change.