fix(resession): fix vim.t.bufs arrays restoring as non-contiguous

[csterlent]

📑 Description

Astrocore tries to restore all tabs when it loads a session, even the ones that failed resession's buf_tab_filter and did not get saved. When this happens, those missing tabs are placed as nil in the tabpage's vim.t.bufs variable. This can cause error messages, lost tabs, and continuing unexpected behavior.

This PR will make those nil values be ignored, so the array indexes are contiguous.

It is still possible to load in and the current buffer be some unlisted bufnr because of the buf_tab_filter. I recomend that vim.cmd.bf is called somewhere if that happens. Additionally, I noticed that tab-scoped sessions seem to load in without a tabline at the top, with or without the change in this PR.

📖 Additional Information

I attached an error log caused by adding two tabs that get filtered out on session save
:TSModuleInfo
:TSModuleInfo
:q
(Reopen)
Sl
error.log

[mehalter]

Thanks! This is a great improvement