Q:

Valid MAC-Address with a zero-byte at last position? 

is it possible that a valid MAC-Address has a empty (zero) byte at the last possition? 
For Example:
Byte 1 = 00
Byte 2 = 26
Byte 3 = B9
Byte 4 = 13
Byte 5 = 7B
Byte 6 = 00

A:

Yes, in fact 1 in 256 addresses will have that feature.