Are I2C Transactions with no data (SDA H->L->H with SCLK high) legal?
From I2C specifications:

A START condition immediately followed by a STOP condition (void message) is an illegal format. Many devices however are designed to operate properly under this condition.

So, as per the specs, it is illegal, but in practice, it depends on the chips on the bus. Some I2C peripherals of MCUs do not like sequences that are not by the protocol, even though there is nothing wrong in theory about sending START and STOP.