Fixed regression caused by the defaultValue feature

[1] added support for selecting default values for labels.
Unfortunately it also broke the ability to remove labels
as described in [2], more precisely by: "To remove a label in a child
project, add an empty label with the same name as in the parent."

This fix make it possible to push empty labels in project.config
to refs/meta/config again. Without it, the following error is returned
by gerrit: 'project.config: Invalid defaultValue "0" for label ...'

[1] https://gerrit-review.googlesource.com/#/c/55750/
[2] https://gerrit-review.googlesource.com/Documentation/config-labels.html#label_custom

Change-Id: Icd727f64cb7a904957a2acc5143fe801653cfabe
1 file changed