This is a rhetorical question, that is, a question asked in order to make a statement instead of elicit information in reply. An analogous question is in what Paul says in Romans 8:32: “If God is for us who can be against us.”
The rhetorical question expresses that there is not harm, usually, for those who do good. Therefore, the implied answer in that question is a definite “No, no one will be able to harm you if you are zealous for what is good.”1
13 Now who is there to harm you if you are zealous for what is good?