Has there ever been a tail-dragger with retractable tail-gear?
Yes, there are. The Boeing B-17 FLying Fortress is one example of an aircraft designed with a retractable tailwheel.


Source: USAAF via Wikimedia, Public Domain (USGOV-PD)

Another example would be the Vought F4U Corsair:


Source: Wikimedia, cc-by-sa-2.0

Doubtless there are also other examples of aircraft similarly equipped with retractable tailwheels.