fix: don't display picker when no build targets are returned

When you just opened up a project if you quickly tried to get the build
target info before importing was done, you'd get an empty table back
resulting in an empty picker, which was confusing. This just adds in a
check to ensure the table isn't empty.

lua/metals.lua

@@ -358,7 +358,7 @@ M.show_build_target_info = function()
     if err then
       log.error_and_show(string.format("Unable to retrieve build targets", method))
       log.error(err)
-    elseif resp then
+    elseif resp and not vim.tbl_isempty(resp) then
       vim.ui.select(resp, {
         prompt = "Choose build target to view:",
       }, function(choice)
@@ -374,7 +374,7 @@ M.show_build_target_info = function()
         end
       end)
     else
-      log.warn_and_show("Metals returned no info for build target.")
+      log.warn_and_show("Metals returned no info for build targets. If you're still importing, wait and try again.")
     end
   end
   execute_command({ command = "metals.list-build-targets" }, choose_and_decode)