Multiply cells only if a cell is nonblank

In the attached spreadsheet, I wish to run the below calculation in DATA J2. However, I only want (I2+H2)/2 to occur if H2 is nonblank. If H2 is blank, then only calculate (=I2-E2).

=((I2+H2)/2)-E2

Basically, I want to...

Calculate ((I2+H2)/2)-E2 if H2 is nonblank, otherwise calculate =I2-E2