(PUP 229) Fix /etc/shadow parsing so that max/min_age is reported correctly

Before this patch, parsing /etc/shadow, when empty trailing fields were
present, they were discarded, and inturn a nil check was used to ensure that
the fields did not exist. However, this ran into trouble when a value was
appended to the end, causing all the empty fields to be returned as empty
strings instead, failing the nil checks.

This patch ensures that all empty fields are returned as empty strings, and
a check for empty string is used to check whether the field exists or not.
Merge pull request #2975 from vrthra/fix/pup229/maxage

(PUP 229) Fix /etc/shadow parsing so that max/min_age is reported correctly