empty jshintrc, make sure we use the jshint defaults

On Jenkins, jshint will fallback using the jshintrc provided in
integration/docroot.git Providing an emtpy jshint will make sure Flow
will use whatever jshint default rules are.

See bug 52456

bug: 54651

Change-Id: I29bed83d155d9882fe8e8f954b67aac2eb0e25a9