[backport#15337] rpc: Fix for segfault if combinepsbt called with empty inputs

Summary:
30d0f7be6e rpc: Fix for segfault if combinepsbt called with empty inputs (benthecarman)

Pull request description:

Fixes #15300

Backport of Core PR15337

Test Plan:

ninja
test_runner.py rpc_psbt

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6590