Return null in getBestStatements if there are none

wdtk-datamodel/src/main/java/org/wikidata/wdtk/datamodel/implementation/StatementGroupImpl.java

@@ -99,6 +99,7 @@ public StatementGroup getBestStatements() {
 				bestStatements.add(statement);
 			}
 		}
+		if (bestStatements.size() == 0) return null;
 		return new StatementGroupImpl(bestStatements);
 	}
 

wdtk-datamodel/src/main/java/org/wikidata/wdtk/datamodel/interfaces/StatementGroup.java

@@ -45,7 +45,7 @@ public interface StatementGroup extends Collection<Statement> {
 	 * These are the statements with rank {@link StatementRank::PREFERRED }
 	 * if they exists or the one with rank {@link StatementRank::NORMAL }
 	 *
-	 * @return a subset of the current StatementGroup
+	 * @return a subset of the current StatementGroup, or null if there are no best statements
 	 */
 	StatementGroup getBestStatements();
 

wdtk-datamodel/src/test/java/org/wikidata/wdtk/datamodel/implementation/StatementGroupTest.java

@@ -44,6 +44,8 @@ public class StatementGroupTest {
 			Collections.emptyList(), Collections.emptyList(), subject);
 	private Statement statementEmptyId = new StatementImpl("", StatementRank.NORMAL, mainSnak,
 			Collections.emptyList(), Collections.emptyList(), subject);
+	private Statement statementDeprecrated = new StatementImpl("DepId", StatementRank.DEPRECATED, mainSnak,
+			Collections.emptyList(), Collections.emptyList(), subject);
 	private StatementGroup sg1 = new StatementGroupImpl(Collections.singletonList(statement1));
 	private StatementGroup sg2 = new StatementGroupImpl(Collections.singletonList(statement1));
 
@@ -79,6 +81,14 @@ public void getBestStatementsWithoutPreferred() {
 		);
 	}
 
+	@Test
+	public void getBestStatementsEmpty() {
+		assertNull(
+				new StatementGroupImpl(Collections.singletonList(statementDeprecrated)).getBestStatements()
+
+		);
+	}
+
 	@Test
 	public void propertyIsCorrect() {
 		assertEquals(sg1.getProperty(), property);