PUD: nmealib: fix parsing of date in GPRMC

It appears that the field is optional/can be empty and that parsing
such an empty field would fail.

Signed-off-by: Ferry Huberts <f.huberts@mindef.nl>
pud: nmealib: fixup the previous commit

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>