I would send this to FileMaker via the Feedback form, but the Feedback form only allows reports about the Documentation. This is a bug report, not a report about the documentation.
If you use the SendMail function in FileMaker Advanced 12.0 v2, with perform without dialog UNCHECKED, and you do not specify any message content, there is no message created in the Email client. If you type even a single character in the message text, then the expected behavior occurs, which is that the message appears as an open window.
The consequence is that if you want to give the user a simple button to open a pre-addressed message in which they can write their own content, it can't be done unless you fool the system with a space character or something else like "Write your message here."
The exact same behavior occurs when using this function as a script step: the email client becomes active, but no message is created.