Allow NULL default values in @Plugin annotations

This issue is being reviewed by the kind folks in Slack, #needs-review-queue-initiative. We are working to keep the size of Needs Review queue [2700+ issues] to around 400 (1 month or less), following Review a patch or merge request → as a guide.

Didn't see a test patch so running locally

d6/FieldDiscoveryTest

Failed asserting that two arrays are identical.

d7/FieldDiscoveryTest

Failed asserting that two arrays are identical.

PluginTest actually passes. (not sure an issue)

Reviewing the fix

-    // Only keep actual default values by ignoring NULL values.
-    $defaults = array_filter($reflection->getDefaultProperties(), function ($value) {
-      return $value !== NULL;
-    });
+    $defaults = $reflection->getDefaultProperties();
+    unset($defaults['definition']);

Appears to be doing what the IS states.

Looks good to me.

PluginTest failing without the fix is a blocker IMO. The other two are test tweaks needed to adapt to this change, but the unit test for this code should absolutely cover this.