Congress referred the charges to the DOJ and it's now up to them. If there were no penalty for refusing to give testimony under oath in front of the commission when there is a lawful reason to compel that testimony, then the investigatory powers of Congress to fulfill their duties would be neutered.
You do understand that the "nothing burger" was actually something that Trump only avoided because he was Prez at the time? In the grand scheme of things, he may or may not stand for the crimes the Muellar Commission found Trump had committed. In this context, nothing burgers are wishful thinking of the criminal that he won't be caught and punished.
This doesn't sound at all trivial:
The criminal offense of contempt of Congress sets the penalty at not less than one month nor more than twelve months in jail and a fine of not more than $100,000 or less than $100. Not unless you think a month in prison is nothing.