maybe_command(): Ensure that command is defined and non-empty

[jkeenan]

As demonstrated in https://github.com/cpan-testers/CPAN-Reporter/issues/59#issuecomment-1753729292, it is possible to exercise subroutine maybe_command() without a defined argument. We should preclude this from happening.

[haarg]

This doesn't look right to me. The current maybe_command method is signaling failure by returning false, but this is making it sometimes throw an exception.

[jkeenan]

This doesn't look right to me. The current maybe_command method is signaling failure by returning false, but this is making it sometimes throw an exception.

Correct. I have revised the subroutine call and the tests. Please re-review.