Use empty hash fallback when loading state file in replicate-changesets

[AntonKhorev]

If I use readme at https://github.com/zerebubuth/openstreetmap-changeset-replication as a guide, it says "a state file, which can be empty the first time it's used". But that's not going to work because an empty file is read as nil, then the code tries to access elements of that nil and crashes.

Here I change an empty state_file to be read as an empty hash.

[tomhughes]

In practice though it's probably necessary to create one manually to bootstrap - we didn't start from the first changeset when we started publishing the changeset replication feed for example.

What does the code actually do when given an empty hash? Start replicating from the first changeset?

[AntonKhorev]

I didn't get the code fully working yet, there are utc vs local timezone issues. But looks like it won't start from the first changeset.