use default prune config if prune=true and not set

Author: markjhoy

server/src/main/java/org/elasticsearch/index/mapper/vectors/SparseVectorFieldMapper.java

@@ -516,7 +516,7 @@ public static Boolean parseIndexOptionsPruneValue(Map<String, Object> indexOptio
         public static PruningConfig parseIndexOptionsPruningConfig(Boolean prune, Map<String, Object> indexOptionsMap) {
             Object pruningConfiguration = indexOptionsMap.remove(IndexOptions.PRUNING_CONFIG_FIELD_NAME);
             if (pruningConfiguration == null) {
-                return null;
+                return new PruningConfig(PruningConfig.DEFAULT_TOKENS_FREQ_RATIO_THRESHOLD, PruningConfig.DEFAULT_TOKENS_WEIGHT_THRESHOLD);
             }
             if ((pruningConfiguration instanceof Map) == false) {
                 throw new MapperParsingException("[index_options] field [pruning_config] should be a map");