Add API for asking whether the pool is empty

[aleixpol]

On certain systems a refresh is necessary in such cases and, in any case, it's useful to know whether queries are pointless.

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I wonder if it would make sense to be more specific about what we are looking up, since I guess it could be getting confused by fwupd or flatpak...

[ximion]

Quite a few changes needed.
AsCache should expose a more low-level API, so returning a component-count makes a lot of sense here, as we will probably want to have that value returned by the pool anyway as well (so a software center can also refresh if the amount of components looks suspiciously low). but the is_empty function can be built upon that.
Also, so _get_ prefix is needed for _is_*-style functions.

The way emptyness is determined is super inefficient here, but if you want to I could fix that up for you in a follow-up commit once this PR is merged (at the moment we deserialize all data and then throw it away just to determine the amount of stuff in the cache, so we at least need to stop doing that. But there's even more efficient methods to do determine the amount of cache entries).