BigQuery: Fix NPE for null table schema fields (#4338)

* Add failing test for empty table schema

* Fix NPE if table schema returns null for fields

Author: chingor13

google-cloud-clients/google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/Schema.java

@@ -18,10 +18,13 @@
 
 import static com.google.common.base.Preconditions.checkNotNull;
 
+import com.google.api.services.bigquery.model.TableFieldSchema;
 import com.google.api.services.bigquery.model.TableSchema;
 import com.google.common.base.Function;
 import com.google.common.base.MoreObjects;
 import java.io.Serializable;
+import java.util.Collections;
+import java.util.List;
 import java.util.Objects;
 
 /** This class represents the schema for a Google BigQuery Table or data source. */
@@ -90,6 +93,10 @@ TableSchema toPb() {
   }
 
   static Schema fromPb(com.google.api.services.bigquery.model.TableSchema tableSchemaPb) {
-    return Schema.of(FieldList.fromPb(tableSchemaPb.getFields()));
+    List<TableFieldSchema> fields = tableSchemaPb.getFields();
+    if (fields == null) {
+      fields = Collections.emptyList();
+    }
+    return Schema.of(FieldList.fromPb(fields));
   }
 }

google-cloud-clients/google-cloud-bigquery/src/test/java/com/google/cloud/bigquery/SchemaTest.java

@@ -18,6 +18,7 @@
 
 import static org.junit.Assert.assertEquals;
 
+import com.google.api.services.bigquery.model.TableSchema;
 import com.google.common.collect.ImmutableList;
 import java.util.List;
 import org.junit.Test;
@@ -57,4 +58,11 @@ private void compareTableSchema(Schema expected, Schema value) {
     assertEquals(expected, value);
     assertEquals(expected.getFields(), value.getFields());
   }
+
+  @Test
+  public void testEmptySchema() {
+    TableSchema tableSchema = new TableSchema();
+    Schema schema = Schema.fromPb(tableSchema);
+    assertEquals(0, schema.getFields().size());
+  }
 }