Empty Block doesn't show up because the build array is empty

I'm not sure I understand the point of the test in BlockExampleTest.php::testBlockExampleBasic(). We have a comment saying "Verify that blocks are there. Empty block will not be shown, because it is empty." but the goal is to display an empty block, not to demonstrate that an empty array prevents the block from showing up, right?

With the above patch, the below assertion will thus fail and we'd need to use assertRaw() instead.

$this->assertNoRaw($settings_empty['label'], 'Block empty not found.');

If we really, truly think that behavior is correct, then it'd require more documentation in ExampleEmptyBlock::build() because it's confusing for sure.

Attached patch fixes the test accordingly to what's the assumed normal behavior.

I still don't really understand why we'd prefer an empty array to an empty block content (especially since it was an empty block content before) but here's another approach with better documentation to hopefully clarify the intent.