src: don't crash with no arguments to print

[cjihrig]

This commit prevents a segfault on v8 print.

I'll be adding a test as well, but it requires some changes to test/common.js first.

[cjihrig]

Replaced the remaining uses of NULL and updated the other commands in a similar fashion. I've got four test cases to add, but have to update the test Session to expose stderr first.

[cjihrig]

OK, I allowed stderr to be scripted the same way that stdout currently is. Then, I added a test for the usage messages. Those commands segfault on master, and work correctly with the fixes in this PR.

[cjihrig]

@bnoordhuis what is the policy for PRs getting landed in this repo? I technically have commit rights and can land this myself, but I don't want to step on any toes either.

[bnoordhuis]

Go ahead and land it, no toes to worry about.

[hhellyer]

@bnoordhuis @cjihrig - I can land things but I'm not sure whether I should be able to in a Node.js repo or whether my access ended up being grandfathered in when @indutny moved the repo from his space to the nodejs org. So I don't usually unless the PR has had a couple of LGTMs and been waiting a little while!
Things are building up though - I'm happy to clean up the backlog if you guys want.

[bnoordhuis]

@hhellyer Go for it! :-)