fix: fallback to last buffer if window empty (#66)

[jpetrie]

If curwin is nil during session load (which can happen when the buffer in the only window was filtered out of the session), fall back to opening the last restored buffer.

Currently, as described by #66, if the last buffer in the last window is something you're filtering out (by default or with a custom buf_filter, when you restore that session you get an empty window. With this change, you'll instead get the most-recently-restored buffer from the session.

This should only affect single-window, single-tab scenarios (the existing behavior in those scenarios, where the window containing the filtered-out buffer is simply not restored) should be preserved by this change.

This isn't gated by an option because the current behavior felt like a bug to me, but it would be pretty easy to add a fall_back_to_most_recent option (maybe with a better name) if desired.

[stevearc]

Agree that this is better default behavior. Thanks for the PR!