Signer should return exit code 0 when run with no args

stacks-signer when run with no args simply displays a help message, and then exits with code '2'.

However, when using an arg like --help or --version, simialr output is displayed and the exit code is 0 (expected).

the exit code of 2 makes it challenging to check the binary is there in some helper scripts:

$ stacks-signer > /dev/null 2&>1; echo $?
2
$ stacks-node > /dev/null 2&>1; echo $?
0

Propose that if no args are defined, keep behavious as it is today but return an exit code of 0