Selection with partially empty results

How can I create a TQL query that returns also empty results for certain properties in the querie´s Selection part?

This is my query:

lu, as 's:IssueName', as 's:IssueStatus':
s.beginTime == in {"now/M"}, exists t.e1.userInput

It returns only results for transactions which make use of the metadata tags ISSUE_Name and ISSUE_Status; I would like to display all transactions, with empty results in case these metadata tags have not been used inside a transaction.

Yes, you can do this by using the exclusion operator ! , you will then see (null) as result for the metadata tags if they are not present in a transaction.
The default Inquire behaviour is to remove nulls and undefined results in order to improve the performance of your query since in many use cases these results would be meaningless anyway. For use cases, as yours, where you need to display those null results, you have then the exclude operator available.
Your query would look then like this:

lu, ! as 's:IssueName', ! as 's:IssueStatus':
s.beginTime == in {"now/M"}, exists t.e1.userInput

You can find more detailed info here: Document