@Override
    public void setSecondPhaseRanking(String expression) {
        // Ignore
    }
A rank profile which ignores all calls made to it which may fail in a document only setting.
This is used by the search definition parser when it is requested to parse documents only,
to avoid having to check for this in every method which adds to the rank profile.
(And why do we ever want to parse documents only? Because it is used when generating Java classes
from documents, where the full application package may not be available.)

@author bratseth
config-model/src/main/java/com/yahoo/schema/DocumentsOnlyRankProfile.java