There's nothing "magic" about hibernate. When your system boots up, the OS initializes whatever it needs to the get to a state in which it can boot, then checks to see if their is a hibernation file. If there is, it will load it into RAM, and do whatever it needs to bring the system into an operational state. If not, the OS will continue the boot process. Regardless, decryption of your disk occurs long before the system has even checked for the existence of a hibernation file, let alone needed to start using its contents.