fix: only filter msg if sev is hardcoded (#328)

Fixes previous breaking change that prevented nvim severity tables from
working, such as:
require("trouble").setup({
	severity = { min = vim.diagnostic.severity.INFO },
})

Optimal fix might be a less naive way of filtering severity, but I'd
leave that for somebody that cares about this feature.

Co-authored-by: Willem Jan Noort <noortw01@heiway.net>

lua/trouble/providers/diagnostic.lua

@@ -31,7 +31,7 @@ function M.diagnostics(_, buf, cb, options)
   end
 
   local messages = {}
-  if options.severity ~= nil then
+  if severity[options.severity] then
     table.insert(messages, { text = "filter:", group = "Information" })
     table.insert(messages, { text = severity[options.severity], group = "Sign" .. util.severity[options.severity] })
   end