This test would previously fail without a user-friendly warning message, if invoked with no arguments.
Test plan:
bitcoin @_@$ python3 test/lint/check-rpc-mappings.py
Usage: test/lint/check-rpc-mappings.py ROOT-DIR
bitcoin @_@$ echo $?
1
bitcoin @_@$ python3 test/lint/check-rpc-mappings.py .
* Checking consistency between dispatch tables and vRPCConvertParams
bitcoin @_@$ echo $?
0