Would this be the first time that there is no GOP debate because no one can run against Trump?
Primary debates (and elections) are held to weed out a party's field of candidates running to be the nominee.
If the incumbent President is running for another term, they are always the presumptive party nominee. It would be foolish for another candidate to run for the party nomination unless the incumbent could be easily beaten.
Primaries are run by the party in each state. There is nothing that says a primary election is even required if the party delegates have already decided the incumbent is their nominee. That means there's no way for another candidate to win the nomination. Unless the convention becomes contested, meaning somehow the incumbent failed to get more than 50% of delegates to support him, the nomination is a lock.
If the GOP primaries are held, and no GOP candidate gets more than 50% of the delegates, then the convention becomes contested. They hold round after round of voting by delegates. The first round, they must vote by their state's primary results. Some states award all delegates to the winner of their primary election, and others award a proportion of delegates to each candidate based on the % of votes received. After that round of voting, if no candidate gets >50% of delegates' votes, they vote again, this time unbinding some of the states' delegates from their primary election results. If that fails to name a winner, they hold another round unbinding more delegates. Basically, they let more and more delegates vote their preference versus voting the way the state's primary voters wanted them to vote.
Eventually when a candidate reaches >50% of delegates, a nominee is named.
So, if the party doesn't hold primary elections, all delegates are free to vote their preferences. Hypothetically, if REAL evidence of a REAL crime is made public that the incumbent needs to be impeached over, and impeachment takes longer than the date of the convention, it's possible another candidate can win over enough delegates to replace the incumbent as the nominee -- no debates, no primary campaigning, and no significant fundraising to that point.