Fix empty FindNearest pb instance

Author: cindy-peng

google-cloud-datastore/src/main/java/com/google/cloud/datastore/FindNearest.java

@@ -191,6 +191,7 @@ public enum DistanceMeasure {
      * unit normalized vectors instead of COSINE distance, which is mathematically equivalent with
      * better performance.
      */
+    DISTANCE_MEASURE_UNSPECIFIED,
     COSINE,
     /** Measures the EUCLIDEAN distance between the vectors. */
     EUCLIDEAN,

google-cloud-datastore/src/main/java/com/google/cloud/datastore/StructuredQuery.java

@@ -917,7 +917,7 @@ B mergeFrom(com.google.datastore.v1.Query queryPb) {
       for (com.google.datastore.v1.PropertyReference distinctOnPb : queryPb.getDistinctOnList()) {
         addDistinctOn(distinctOnPb.getName());
       }
-      if (queryPb.getFindNearest() != null) {
+      if (queryPb.getFindNearest() != null && queryPb.getFindNearest() != com.google.datastore.v1.FindNearest.getDefaultInstance()) {
         setFindNearest(FindNearest.fromPb(queryPb.getFindNearest()));
       }
       return self();