Improve text for empty issue/pr description (#26047)

The previous text implied that content was going to be added later, but
I don't think we should make such assumptions.

<img width="346" alt="image"
src="https://github.com/go-gitea/gitea/assets/115237/518133a7-2c4c-4463-91ee-5fe439898032">

Author: silverwind

options/locale/locale_en-US.ini

@@ -1448,7 +1448,7 @@ issues.context.quote_reply = Quote Reply
 issues.context.reference_issue = Reference in New Issue
 issues.context.edit = Edit
 issues.context.delete = Delete
-issues.no_content = There is no content yet.
+issues.no_content = No description provided.
 issues.close = Close Issue
 issues.comment_pull_merged_at = merged commit %[1]s into %[2]s %[3]s
 issues.comment_manually_pull_merged_at = manually merged commit %[1]s into %[2]s %[3]s