Fix bug in minmax normalizer

Author: Mikep86

x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/linear/MinMaxScoreNormalizer.java

@@ -56,7 +56,9 @@ public ScoreDoc[] normalizeScores(ScoreDoc[] docs) {
         for (int i = 0; i < docs.length; i++) {
             float score;
             if (minEqualsMax) {
-                score = min;
+                // This can happen if there is only one doc in the result set or if all docs have nearly equivalent scores
+                // (i.e. within epsilon). In this case, assign every doc the max normalized score.
+                score = 1.0f;
             } else {
                 score = (docs[i].score - min) / (max - min);
             }