fix(todo_comments): handle nil parser from get_parser on nvim 0.12 (#338)

Since neovim/neovim#37276 (released in 0.12), vim.treesitter.get_parser()
returns nil instead of throwing when no parser is installed for the
language. The pcall therefore succeeded with parser == nil, and the
subsequent parser:parse() errored with "attempt to index local 'parser'
(a nil value)" on any buffer whose filetype has no treesitter grammar.

Drop the pcall and check the return value directly. Pass
`{ error = false }` so 0.11 also returns nil instead of throwing, on
0.12+ the option is ignored.

Author: r-vdp

lua/null-ls/builtins/diagnostics/todo_comments.lua

@@ -18,8 +18,10 @@ local function get_document_root(bufnr, filetype)
         return
     end
 
-    local has_parser, parser = pcall(vim.treesitter.get_parser, bufnr, lang)
-    if not has_parser then
+    -- `error = false` is only needed for nvim 0.11; on 0.12+ get_parser never
+    -- throws and returns nil when no parser is available.
+    local parser = vim.treesitter.get_parser(bufnr, lang, { error = false })
+    if not parser then
         log:debug("no parser available for lang " .. lang)
         return
     end