src/pcrlock/pcrlock.c: Handle empty pcrlock.d directories

src/pcrlock/pcrlock.c: Handle empty pcrlock.d directories

Running the following commands:

  # mkdir -p /var/lib/pcrlock.d/123-empty.pcrlock.d
  # /usr/lib/systemd/systemd-pcrlock predict --pcr=1+2+3+4+5+16

Will result in:

...
Floating point exception

Running the following commands:
  # mkdir -p /var/lib/pcrlock.d/123-empty.pcrlock.d
  # /usr/lib/systemd/systemd-pcrlock make-policy --pcr=1+2+3+4+5+16

Will result to this (partial) log:
...
Predicted future PCRs in 133us.
[]
...
Written policy digest 0000000000000000000000000000000000000000000000000000000000000000 to NV index 0x1921da6
...

So, add missing checks to handle gracefully cases where there's no variant
inside the component.

Signed-off-by: Arnaud Patard <arnaud.patard@collabora.com>