Ignores empty property values from CSV input

by adding an additional CharSeeker#tryExtract returning whether or not a
value was extracted. Extractor#extract was changed to return whether or
not a values extracted for this occasion as well. The existing
CharSeeker#extract method throws exception if no value was extracted.
  Therefore skipping of empty values is consistent, whereas previously
non-primitive extractors could return null which was the only tell whether
or not a value was extracted.