I think the most popular answer is some variation on the one the Noam Elimelech writes in the name of his brother (posted about it here). A true ba’al bitachon asks no questions. He trusts that if Hashem says not to farm, he will survive in some way. That’s exactly what will happen – someway, somehow, the food will last. The teva cannot and will not contradict the belief of the ba’al bitachon. What the Torah here is saying is that even if you do ask questions and therefore maybe you don’t deserve teva to be in harmony with your shmiras hamitzvos, you don’t deserve the food to last, still Hashem promises that so long as you are willing to do the mitzvah, he will force the food to be there by commanding his bracha to the fields.
I want to share with you the answer of the Maharasham (I saw this quoted by R’ Shmuel Ya’akov Rubenstein in his She’eiris Menachem, where he adds additional hesber that I am not going into, so here’s a link). There is another place in the Torah where a promise appears in the context of a question and answer: the Torah writes (Devarim 7:17) that if you should say when you enter Eretz Yisrael, “How can I conquer all these great nations?” don’t worry because Hashem promises that He will help get the job done. Again, one can ask why the promise is couched in the context of shakla v’terya instead of being said straight out. The Bina La’ittim answers (and I once quoted the same pshat from Sefas Emes) that the Torah is teaching that Hashem’s promise is conditional on our asking for it. Meaning, we have to first recognize that we can’t do it ourselves; the job is too great and our ability is limited. We have to say to Hashem that we need him. If we do that, then Hashem promises to deliver the help we need. If think we can do it ourselves, we fail to recognize our own limitations, then Hashem leaves it up to us, and of course, we can’t really succeed by ourselves. The same is true of shemita. The Torah prefaces the promise of Hashem's help with the farmer's question to teach us that Hashem will provide us with all the help we need to get through the years without farming – but we have to recognize that we need that help and ask for it.