🐛 Fixed admin error when deleting an unsaved or imported post

[cmraible]

ref https://linear.app/tryghost/issue/ENG-845/error-attempted-to-set-lexical-on-the-deleted-record
ref https://linear.app/tryghost/issue/ENG-854/🐛-deleting-imported-posts-makes-ghost-unresponsive

• When deleting a post in the editor's Post Settings Menu, if the post has unsaved changes (indicated by the hasDirtyAttributes property in the editor), Admin will crash because it tries to save a post revision before leaving the editor, but the post has already been deleted so saving fails.
• This can occur when editing a post and quickly deleting it from the Post Settings Menu before saving is completed.
• It can also occur when attempting to delete an imported post, as the editor will parse the lexical from the server and may make some minor, invisible-to-the-user changes to the lexical string locally (e.g. JSON formatting, or updating the JSON to use extended version of base lexical nodes), which triggers the same error.
• This fix bypasses the attempt to save a post revision when leaving the editor if the post is already deleted, which allows the transition back to the Posts route to succeed.