Note, The `fileio_linux_amd64` test binary was manually copied from the
examples directory and added to Git.  Maybe one day we will come up with
a more principled mechanism for this.

Also note, this test case currently fails, even with the `sys_open` type
fixes.  Multiple errors are revealed by this test case.

This function never seems to be used without specifying the formerly
optional value.

This is an expedient fix -- a more proper fix would nail down the API of
which characters are allowed in keys, maximum length etc, at the level
of the `Store` class.

Our GitHub CI has one step where it builds each of the example programs,
and then attempts to run them with no arguments and no standard input,
checking that they exit with code 0.  Rather than rework this approach
to support more general invocation patterns for example programs, it's
easier to change the `fileio` example I added.