Do not add a condition when missing filter has no values

2 test failures are coming from 2.0.x branch - fixing coding standards issue

At this stage it's probably better to target 3.x branch.

+++ b/src/Plugin/facets/query_type/SearchApiString.php
@@ -47,7 +47,10 @@ class SearchApiString extends QueryTypePluginBase {
+          if ($value === '!()') {

This needs some explanation.

Added comments - in doing so I moved the condition inside the original first if statement.

I originally had it outside to avoid too many nested levels in the method - but reading the code with the comments back made me think its more logically placed inside the existing condition to keep the logic around missing item values in one place / more readable.

Moved patch to MR

Ci shows test passing + test only job failing to demonstrate the error.

Lint jobs - lots of errors / warnings on 3.0.x branch for most jobs - does not appear from a quick look to be introducing anything new here.