[rpc] mining: Omit uninitialized currentblockweight, currentblocktx

Summary:
This is a backport of Core PR15383

Obviously currentblockweight => currentblocksize for us.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5945