Werk #15698: mk_filestats: Crash on baking with "Deploy without configuration"

This is a regression that came with Checkmk 2.1.0p26/Werk #15458.

When configuring the agent ruleset Count, size and age of files - mk_filestat (Linux/Solaris) and selecting "Deploy the Filestats plugin without configuration", previously, the agent bakery did crash with a message containing

cmk.utils.exceptions.MKGeneralException: Error running automation call bake-agents (exit code 2), error:
‘bool’ object is not iterable

As a consequence of the fix, the structure of the ruleset had to be adapted to actually support rule merging.
See the inline help of the ruleset for more details.
However, existing rules will be auto-migrated to the new structure.

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