doc: Fix rpc docs
Summary:
The fix removes the "type" OBJ_EMPTY added in commit 8d1a3e6, which isn't really a separate type and instead runs a check on OBJ whether it is empty or not.
Backport of core#24155.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Subscribers: PiRK
Differential Revision: https://reviews.bitcoinabc.org/D16248