The fluid is checked at operating temperature, the only way to get it to operating temperature is to drive the car around for 15-20 minutes.

Once the fluid is at operating temp, you check the level with engine running and tranny in park. Make sure you're on a level surface for the reasons said above.

If you're getting NO fluid on the dipstick when the transmission is up to temp and the engine running, you're seriously low on fluid. Frankly I'm surprised you aren't having flaring issues or gear engagement problems. It's worth noting that the fluid thins out quite a bit when hot. It can be difficult to see on the dipstick.

A '79 will likely have a 722.1 transmission too, not the 722.3.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)