FIX Class manifest is not caching enums

[lekoala]

Description

Fix #11531

Enums were not being added to cache, but validated and required

Manual testing steps

See related issue

Basically, this will ensure that validateItemCache will actually return true
Currently it will always return false unless i've missed something somewhere...

Issues

Fixes #11531

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[lekoala]

Actually now i see why enums are not there

silverstripe-framework/src/Core/Manifest/ClassManifest.php

Lines 694 to 701 in f40b1a4

	if ($changed && $this->cache) {
	$cache = [
	'classes' => $classes,
	'interfaces' => $interfaces,
	'traits' => $traits,
	];
	$this->cache->set($key, $cache);
	}

[lekoala]

For reference, it's due to this PR
d3f1043

which seems to have forgotten to store the enums in the cache

@kinglozzer did I miss something here?

[kinglozzer]

I can’t remember (or think of) any reason why they shouldn’t be included in the cache, so it’s probably just an oversight in the original PR

[lekoala]

an additionnal improvement would be to not store empty arrays and simply consider missing keys as empty arrays.
this should reduce the cache size by a large margin, because it's currently storing a lot of empty traits => [] and enums => []