Filebeat introduces a meta field to registry entries in 6.3.1. The meta field is used to distuingish different log streams in docker files. For other input types the meta field must be null. Unfortunately the input loader did initialize the meta field with an empty dictionary. This leads to failing matches of old and new registry entries. Due to the match failing, old entries will not be removed, and filebeat will handle all files as new files on startup (old logs are send again).

Users will observe duplicate entries in the reigstry file. One entry with "meta": null and one entry with "meta": {}. The entry with "meta": {} will be used by filebeat. The null-entry will not be used by filebeat, but is kept in the registry file, cause it has now active owner (yet).

Improvements provided by this PR:

* when matching states consider an empty map and a null-map to be equivalent
* update input loader to create a null map for old state -> registry entries will be compatible on upgrade
* Add checks in critical places replacing an empty map with a null-map
* Add support to fix registry entries on load. states from corrupted 6.3.1 files will be merged into one single state on load
* introduce unit tests for loading different registry formats
* introduce system tests validating output and registry when upgrading filebeat from an older version

Closes: #7634

(cherry picked from commit c558984e29dc04340d1dc129f6020189d5d26466)