Remove double dots from cache path without keys

When no key is specified, do not concat the string with two dots
as it results in a double dotted file path for both cache files.
Adding tests to cache files without config cache key.
#6552 - adding missing `@group` annotations for newly introduced tests
Merge branch 'hotfix/#6552-remove-double-dot-in-merged-config-cache-file-name' into develop

Close #6552