I do not know the frequency of the issue, so the following ideas are a bit blind and may not be good for a ROI.
Some alternative ideas to just being able to "delete" wiki and excerpt text.
Have template text made up and ready for wikis and excerpts.
When a bad edit occurs, edit the tag wiki and excerpt and paste the text in.
This removes the offending text, and (with the right text) encourages any users seeing the template text to improve the wiki and excerpt (better than blank?):
[currently no wiki]
This tag wiki is in need of a really good description.
If you're knowledgeable in this tag, and are familiar with the requirements for tag wikis, please add/suggest a description here.
[currently no excerpt] This tag excerpt is in need of a really good
(Obviously Stack would write them in their words.)
This issue could also be resolvable if there was at least one good Revision History to roll back to prior to a bad edit.
(2) Template auto inserted into blank tag wikis/excerpts
Having templates ready somewhere to be used manually and likely not very often is laborious.
Using my template text above as (crude) "defaults", a Stack dev could automatically insert them into all wikis and excerpts which have never had text, i.e. have no Revision History.
Also, adjust other scripts so whenever a new tag is made it's wiki and excerpt also get the default text.
Then if any bad edits occur, there would be a decent/clean Revision History prior to the bad edit, and can just rollback back to the default texts.
(3) Auto insert blank edits
If using "default text" is not welcomed.
This might be too hacky, but throwing it out there.
Stack devs could insert a blank edit into all wikis and excerpts which have never had text, i.e. have no Revision History.
So they would invoke new database IDs (etc) for the wikis and excerpts and their corresponding Revision History.
So, while the wikis and excerpts themselves will be blank, there will be a genuine Revision History, albeit also blank.
Then if any bad edit occurs, just rollback back to the first Revision History, which is blank - no more bad text.
Again, this one sounds a bit cringe-worthy, but not knowing Stack systems and potential changes, and as this might prompt an idea from someone else, thought I'd throw it out there.