Fix a -Wreturn-type warning with GCC

Summary: The RPCArg::ToStringObj method is supposed to return a string, but in the case of an unused type conversion it will raise an error and not return anything.

Test Plan:
With GCC 12:

ninja

Check the warning is gone.

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12947