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Display help string if the input read from stdin has length 0 or is not valid json

In that case the error will be caught down the line upon parsing so I think we can keep things that way. It would be consistent with the current behavior, both vroom '' and vroom -i missing_file.json currently error with {"code":2,"error":"The document is empty. (offset: 0)"}.

Only read from stdin as input if stdin is not a tty

I'm not sure to get the implications here but I'm not very familiar with this kind of stuff. In what situation would this be an improvement?

I'm not sure to get the implications here but I'm not very familiar with this kind of stuff. In what situation would this be an improvement?

Running vroom from a terminal without arguments would still print the help message (potentially helping new users) and running cat problem.json | vroom would work as expected. The downside is that there is no way to do this via the C++ standard library and we would have to use isatty for unix-like systems and something else for windows.

I think we can live with vroom (without arguments) not displaying the help message, after all it's a case of misuse and there is an appropriate error message. Also we provide the -h flag, which is the way most command-line users would expect to get some help.

Alright, I added this change to the changelog. This is ready to merge, I think