This is not a programming answer, and may be a little off topic, but I released several applications with similar functionality and saw the same thing.
I also talked with several users who sent blank emails, and found out that sometimes they just check various functions of the application, and sometimes they get a feedback page, but donβt want to spend time typing an email on their phone.
I would think of it as a customer service problem and programming problems. I would take the opportunity to keep track of users who send empty emails. (At least now you have your own email address if you do not allow them to submit the form without a body, which you most likely will never receive.) One option is to send a standard email followed by a message that says that you received an empty email from them and would like to hear their feedback on your product. (Perhaps even send them a quick review).
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