Hello,
Okay, is there any OPCode I can write in the empty flash memory to force the system to return to the program whenever these empty locations are accessed?
Well, simple branch. That will set your PC on the desired address with label.
What pops up in my mind is forcing the system to return to the reset vector location, which may be by setting the PC to the reset vector location.
This however will put the device in some unknown state (as the reset was not performed) and you can have multiple errors active.

For opcode details please refer to the EBOOK reference manual:
Table G-1. Primary opcodes (instruction bits 0:5)
Best regards,
Peter