Merge branch 'cs/#6686-zend-form-return-self-fixes'

Close #6686
Cache: fixed 'Undefined index' error in memory adapter on access tags of unregistered namespace
#6878 - adding missing `@group` annotation for newly introduced tests
Merge branch 'hotfix/6878-undefined-index-errors-in-memory-adapter-on-undefined-namespaces' into develop

Close #6878
Forward port #6878