Inserter: Show messaging when no blocks found

[aduth]

This pull request seeks to add messaging when searching using the inserter reveals no available block options.

Before	After

Testing instructions:

Verify that if no blocks are found by searching the inserter menu that a "No blocks found" message is shown instead.

[jorgefilipecosta]

The code looks good to me and things are working as expected.

[jorgefilipecosta]

If the only usage of this logic is when searching it may make sense to make this verification in line 358 where we call render categories for search as it makes things more explicit.

[aduth]

Yeah, I think that makes sense. It's a bit complicated with trying to render the result of this.getVisibleBlocksByCategory inline, since we'd need to both test its non-emptiness and pass as the argument to renderCategories (or call twice, which is wasteful).

Lodash's _.cond could handle this pretty nicely, though maybe a bit less obvious to read?

cond( [
	[ isEmpty, () => (
		<span className="editor-inserter__no-results">
			{ __( 'No blocks found' ) }
		</span>
	) ],
	[ () => true, this.renderCategories ]
] )( this.getVisibleBlocksByCategory( this.getBlockTypes() ) );

[jorgefilipecosta]

This version with lodash cond is nice, but it is not obvious to read as you pointed out. The other possibility would save getVisibleBlocksByCategory in a const before the render but this way we may be computing it without need. I did not think about the complication because of being inline. It looks like our options have their downsides are not better than the current version we have, so I think we can ignore this detail and merge as it is right now.