So many rebuilds? For this attribute you apparently need null and not empty string.

Oh, in that case this can just go into staging. Python 3.10 support is not that important.

(And I think it would need to use optionalAttrs, null is different from unset if I remember correctly.)

null does work. I did test that (just eval for the hash). That's a generic thing somewhere deep, perhaps in stdenv.mkDerivation.

Some attributes have specific handling also for empty list or empty string, as it's ergonomic with lib.optional*, but apparently not this attribute (yet).