[8.x] Added isNotEmpty method to HtmlString

[austenc]

When working with blade components I've often found myself reaching for an ->isNotEmpty() method on the default slot. This PR adds that method to HtmlString so that it feels more in line with other Laravel objects like collections.

Before

@if (!$slot->isEmpty())
    <span class="...">{{ $slot }}</span>
@endif

After

@if ($slot->isNotEmpty())
    <span class="...">{{ $slot }}</span>
@endif

The tests may not be necessary, but I decided it was better to submit the PR with tests than without them.

Thanks for considering!

[GrahamCampbell]

I don't think this is necessary. The existing code is already clear, and fewer characters.

[austenc]

@GrahamCampbell couldn't you say that about many of the other isNotEmpty methods that exist in the framework too?

It may not be necessary, but it does make the class feel more congruent with other "emptyable" things in the framework. Two extra characters to have a more expressive statement seems worth it. 🤷