The only possibility for having a table with different severities to
pass to `vim.diagnostics.get()` as opts is when setting a range (limit
and bound), so just check for an empty table when a range is set.
* fix: use nil for the default diagnostics severity
https://github.com/neovim/neovim/pull/24736 has introduced a new syntax
for severity format in `vim.diagnostic.get()`.
Fix#2661
* fix: fallback for 0.9.1
* fix: detect changes only when minor version > 9
