Can you put r-134a in an empty r22 system?

No, you should not mix R-134a with an R-22 system. R-22 and R-134a are different refrigerants with distinct properties and are not compatible. Using R-134a in an R-22 system can lead to system inefficiency, potential damage, and could violate environmental regulations. If you need to switch refrigerants, it's best to properly reclaim the existing R-22 and retrofit the system for R-134a or another compatible refrigerant.