fix: avoid crash when there is no selection

This just changes the guard here to make sure that if the user
doesn't select anything with the telescope extension that instead
of crashing we just do nothing.

Fixes #606

lua/telescope/_extensions/metals.lua

@@ -22,8 +22,8 @@ local pickers = require("telescope.pickers")
 local function execute_command(bufnr)
   local selection = action_state.get_selected_entry(bufnr)
   actions.close(bufnr)
-  local cmd = selection.command
-  if cmd then
+  if selection then
+    local cmd = selection.command
     local success, msg = pcall(cmd)
     if not success then
       vim.api.nvim_notify(msg, 2, {})