ref-filter: fill empty fields with empty values

ref-filter: fill empty fields with empty values

Atoms like "align" or "end" do not have string representation.
Earlier we had to go and parse whole object with a hope that we
could fill their string representations. It's easier to fill them
with an empty string before we start to work with whole object.

It is important to mention that we fill only these atoms that must
contain nothing. So, if we could not fill the atom because, for example,
the object is missing, we leave it with NULL.

Signed-off-by: Olga Telezhnaia <olyatelezhnaya@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>