nvim: Fix which-key conditional usage if not loaded

The only left-over which-key invocation which did not check for its
existence beforehand.

nvim/.config/nvim/lua/plugins/config/lsp.lua

@@ -70,7 +70,9 @@ local function on_attach(client, bufnr)
 		{ buffer = bufnr, desc = "Next error" }
 	)
 
-	require("which-key").register({ ["<localleader>l"] = { name = "+language" } })
+	if require("core.util").is_available("which-key") then
+		require("which-key").register({ ["<localleader>l"] = { name = "+language" } })
+	end
 	map(
 		"n",
 		"<localleader>ld",