kernel-install: do not require non-empty kernel cmdline

kernel-install: do not require non-empty kernel cmdline

When booting with Fedora-Server-dvd-x86_64-30-20190411.n.0.iso,
/proc/cmdline is empty (libvirt, qemu host with bios, not sure if that
matters), after installation to disk, anaconda would "crash" in kernel-core
%posttrans, after calling kernel-install, because dracut would fail
with

> Could not determine the kernel command line parameters.
> Please specify the kernel command line in /etc/kernel/cmdline!

I guess it's legitimate, even if unusual, to have no cmdline parameters.
Two changes are done in this patch:

1. do not fail if the cmdline is empty.
2. if /usr/lib/kernel/cmdline or /etc/kernel/cmdline are present, but
   empty, ignore /proc/cmdline. If there's explicit configuration to
   have empty cmdline, don't ignore it.

The same change was done in dracut:
https://github.com/dracutdevs/dracut/pull/561.