stand/fdt: Avoid bailout when dtbo has no fixups

In the case of a simple dtbo where fragment uses target-path and the overlay
contains no references, /__fixups__ will not be included by either our dtc
or dtc from ports, but the file still has valid fragments to be applied.

Additional testing found that /__symbols__ might also be omitted if it's
empty, which is not necessarily an error.

Reviewed by: gonzo, imp
Differential Revision: https://reviews.freebsd.org/D13663