Avoid creating empty bitmaps on R.BITOP command

[aviggiano]

The current implementation creates empty bitmaps for nonexistent source keys srckeys, which are then freed if necessary, since the API must be consistent with redis' native BITOP command.

    Bitmap* bitmap;
    if (srctype == REDISMODULE_KEYTYPE_EMPTY) {
      // "non-existent keys [...] are considered as a stream of zero bytes up to the length of the longest string"
      bitmap = bitmap_alloc();
      should_free[i] = true;
    }
    else {
      bitmap = RedisModule_ModuleTypeGetValue(srckey);
      should_free[i] = false;
    }
    bitmaps[i] = bitmap;

The presence of a should_free variable is specially disturbing, so I think we need to come up with something better.