I've opened the case which caused the upper document, as you had found it's not possible to left empty the tag value once it's set below 7.4.0.
What we use as a workaround it's to set a fake null tag, something like: "MyTagDictionary:Null"
I will try to better describe the issue I ran into.
I have data table in TWX 6.6.11
This data table has about 100 entries.
Part of them has tag value and part not.
The tags doesn't exist in the application anymore.
Why do I care?
I tried to export the data of the data table and import to another server (Same version) and it caused error saying the tag doesn't exist.
So the safest thing to do is to delete the tags, now I'm not sure what exactly the problem.
Is it not possible to add\update entry without specifying tag as you said - if so, how do I have almost all the entries without tags at all?
When you create the entry ( new entry ) you can create it without tags, but once it has one tag you can't remove it.
If you want to import on the new system, you can temporally create the Dictionary/Tag on the new system this way it will allow you to import the data.
If you want to get 100% ride of the not desired tags on the new system ( and it's on 7.4 or above ) you may export the datatable in a csv format and import it again without the tags.
This is actually what I did - created the dictionary and value, since I didn't have much time to wait.
But still I wish to better understand how is it suppose to behave so I can learn for the next time.
As far as I know Data cannot be exported out of the box as CSV only entities.