Don't prime term cache when no posts are passed

[tyrann0us]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Bugfix

What is the current behavior? (You can also link to an open issue here)
When using a permanent object cache, any cache priming is unnecessary because the items are most likely already in the permanent cache.

To make matters worse, the Tribe\Utils\Taxonomy::prime_term_cache method has a major performance problem when called with an empty $posts array. This is the case, for example, when no future events are to be displayed in the "Events List" widget.

In this part of the method:

tribe-common/src/Tribe/Utils/Taxonomy.php

Lines 160 to 165 in 3be81d6

	$args = [
	'fields' => 'all_with_object_id',
	'object_ids' => $ids,
	'taxonomy' => $taxonomies,
	];
	$terms = get_terms( $args );

the following values are passed:

$args = [
  'fields' => 'all_with_object_id',
  'object_ids' => [], // this is the offending part
  'taxonomy'  => ['post_tag', \Tribe__Events__Main::TAXONOMY],
];
$terms = get_terms($args);

This is highly insufficient for systems with a large number of post tags because all post tags will be fetched and passed to _prime_term_caches(). On a project with ~12,000 post tags, we had ~100k requests to the cache.

What is the new behavior (if this is a feature change)?
If the $posts array is empty (i.e. no events are displayed in the widget), bail early and return an empty array. With ~12,000 post tags, we saw a reduction from ~100k requests to ~20k.

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
I don't expect this to be a breaking change.

Other information:
There's an incomplete sentence in the function's DocBlock: Important to note that.
Tagging some of the maintainers to get visibility: @lucatume, @bordoni, @Camwyn (sorry for the notification spam)