That shouldn't occur.
The CASA "Directions" enabling VA and QF to use 1:37 FA ratio on 737-800s say there must be two pax per row in total. If A/B or A/C were occupied I believe they would comply, even with DEF empty. I've see ABC/DEF blocks empty on flights occasionally - although not for long because I usually moved myself there as soon as the seatbelt sign went off.
Interestingly QF's 1:37 waiver says if you have less than 50 pax total, and only two of them are able-bodied, they need to be seated in 13A/B:
CASA 158/13 - Direction - number of cabin attendants in Boeing 737-800 series aircraft, Qantas Airways Limited