I had that board (though I returned it for a different model) and I can confirm it won't damage the card or MB if you plug in the SATA power cable with only one GFX card.
The SATA power connection is just so the board has an extra source of current for the PCI-Express interface, and helps keep your voltage stable. The amount of current remains the same, unless you tweak it in your MBs BIOS/UEFI.
Thank you. i will connect it and leave it.