chore: form binding trim space (#38978)

Use "binding:TrimSpace" instead of fragile IsEmptyString

And fix a bug in locale's `HasKey`: it should also try the default
language if current language doesn't have the translation key, a new
test is added.

options/locale/locale_en-US.json

@@ -532,6 +532,7 @@
   "form.CommitMessage": "Commit message",
   "form.CommitChoice": "Commit choice",
   "form.TreeName": "File path",
+  "form.Title": "Title",
   "form.Content": "Content",
   "form.SSPISeparatorReplacement": "Separator",
   "form.SSPIDefaultLanguage": "Default Language",
@@ -1418,7 +1419,6 @@
   "repo.issues.filter_no_results": "No results",
   "repo.issues.filter_no_results_placeholder": "Try adjusting your search filters.",
   "repo.issues.new": "New Issue",
-  "repo.issues.new.title_empty": "Title cannot be empty",
   "repo.issues.new.labels": "Labels",
   "repo.issues.new.no_labels": "No labels",
   "repo.issues.new.clear_labels": "Clear labels",