dracut-systemd/dracut-cmdline.sh: Don't error out if there is no root= argument.

dracut-systemd/dracut-cmdline.sh: Don't error out if there is no root= argument.

Thanks to systemd's gpt-auto-generator [0] (which implements the Discoverable
Partitions Specification [1]), it is no longer necessary to always specify the
root= argument.

However, dracut would still refuse to boot if there was no root= argument (or
if it was set to the special value "gpt-auto" [2]). This commit stops dracut
from aborting the boot process in these cases and simply lets systemd do its
magic.

[0] https://github.com/systemd/systemd/blob/v229/src/gpt-auto-generator
[1] https://www.freedesktop.org/wiki/Specifications/DiscoverablePartitionsSpec/
[2] https://github.com/systemd/systemd/blob/v229/src/gpt-auto-generator/gpt-auto-generator.c#L928

(cherry picked from commit 016613c774baf3d30c6425a65ead05d8b55d6279)