Always make calls to `vim.diagnostic.set` call `vim.diagnostic.show`.
This creates an easier to reason about code path and is also less
surprising when users wish to override override `vim.diagnostic.show`
with custom behavior and `vim.diagnostic.set` is called with empty
diagnostics.
Functionally, the end result is the same: when `show` is called with an
empty diagnostics list, it just calls `hide` and then returns, which is
exactly what `reset` does right now.
0f554ef6f8
