LUCENE-5245: Fix MultiTermQuery's constant score rewrites to always return a ConstantScoreQuery to make scoring consistent. Previously it returned an empty unwrapped BooleanQuery, if no terms were available, which has a different query norm

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1526399 13f79535-47bb-0310-9956-ffa450edef68