fix: handle malformed backend responses. (#522)

When you delete a policy tag from a column in BigQuery, rather than
eliding the policyTag message entirely, it will return an empty
message (with no name field).  This change treats such responses
as equivalent to no PolicyTag message being present.

Author: shollyman

google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/PolicyTags.java

@@ -60,6 +60,10 @@ com.google.api.services.bigquery.model.TableFieldSchema.PolicyTags toPb() {
 
   static PolicyTags fromPb(
       com.google.api.services.bigquery.model.TableFieldSchema.PolicyTags tagPb) {
-    return newBuilder().setNames(tagPb.getNames()).build();
+    // Treat a PolicyTag message without a Names subfield as invalid.
+    if (tagPb.getNames() != null) {
+      return newBuilder().setNames(tagPb.getNames()).build();
+    }
+    return null;
   }
 }

google-cloud-bigquery/src/test/java/com/google/cloud/bigquery/PolicyTagsTest.java

@@ -18,6 +18,7 @@
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotEquals;
+import static org.junit.Assert.assertNull;
 
 import com.google.common.collect.ImmutableList;
 import java.util.List;
@@ -46,6 +47,13 @@ public void testBuilder() {
     assertNotEquals(POLICY_TAGS, POLICIES);
   }
 
+  @Test
+  public void testWithoutNames() {
+    com.google.api.services.bigquery.model.TableFieldSchema.PolicyTags PARTIALTAG =
+        new com.google.api.services.bigquery.model.TableFieldSchema.PolicyTags();
+    assertNull(PolicyTags.fromPb(PARTIALTAG));
+  }
+
   @Test
   public void testFromAndPb() {
     assertEquals(POLICY_TAGS, PolicyTags.fromPb(POLICY_TAGS.toPb()));