If the answer to the question, "What is in the pocket?" is "Nothing is in the pocket." is the topic marker が needed after "Nothing" (i.e., "なにもが ..." or just "なにも ...")?
"なにも" Should suffice as a response, but of course this would heavily rely on the context and flow of the conversation. You could play it safer by saying "なにもない" which is a little more foolproof and natural sounding. Perhaps が could be used in this more grammatically complete sense "なにもがない". Keep in mind how polite you wish to speak ^^
I Hope this has answered your question. I know this is more of a comment, but I do not have enough rep to just comment. Sorry