nvim: Check linter existence before setting options

We only change 'markdownlint' arguments if it actually exists in the
nvim-lint 'linters' table.

nvim/.config/nvim/lua/plugins/linting.lua

@@ -19,7 +19,9 @@ return {
 				"mfussenegger/nvim-lint",
 				config = function()
 					require("lint").linters_by_ft = linters
+					if require("lint").linters["markdownlint"] ~= nil then
 						require("lint").linters.markdownlint.args = { "--stdin", "--disable", "MD013", "--" }
+					end
 					vim.api.nvim_create_autocmd({ "BufWritePost", "InsertLeave" }, {
 						callback = function()
 							if not vim.g.disable_autolint then