Spliterator improvments (#297)

* fix for NPE on empty source
* add NPE when action is missing (required by contract)
* declare characteristics

src/main/java/org/gitlab4j/api/PagerSpliterator.java

 package org.gitlab4j.api;
 
+import java.util.Collections;
 import java.util.Iterator;
 import java.util.Spliterator;
 import java.util.function.Consumer;
@@ -14,11 +15,16 @@ class PagerSpliterator<T> implements Spliterator<T> {
 		this.pager = pager;
 		if (pager.hasNext()) {
 			elements = this.pager.next().iterator();
+		} else {
+			elements = Collections.emptyIterator();
 		}
 	}
 
 	@Override
 	public boolean tryAdvance(Consumer<? super T> action) {
+		if (action == null) {
+			throw new NullPointerException("Action is null");
+		}
 		if (elements.hasNext()) {
 			action.accept(elements.next());
 			return true;
@@ -42,6 +48,6 @@ class PagerSpliterator<T> implements Spliterator<T> {
 
 	@Override
 	public int characteristics() {
-		return 0;
+		return SIZED | NONNULL;
 	}
 }

src/test/java/org/gitlab4j/api/PagerSpliteratorTest.java

+package org.gitlab4j.api;
+
+import java.util.Collections;
+
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.junit.MockitoJUnitRunner;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.Mockito.when;
+
+@RunWith(MockitoJUnitRunner.class)
+public class PagerSpliteratorTest {
+
+	PagerSpliterator<Integer> pagerSpliterator;
+
+	@Mock
+	Pager pager;
+
+	@Before
+	public void setUp() {
+		pagerSpliterator = new PagerSpliterator<>(pager);
+	}
+
+	@Test
+	public void shouldAcceptElementFromSource() {
+		when(pager.hasNext()).thenReturn(true);
+		when(pager.next()).thenReturn(Collections.singletonList(1));
+
+		boolean success = pagerSpliterator.tryAdvance(System.out::println);
+
+		assertTrue(success);
+	}
+
+	@Test
+	public void shouldDoNotFailOnEmptySource() {
+		boolean success = pagerSpliterator.tryAdvance(System.out::println);
+
+		assertFalse(success);
+	}
+
+	@Test
+	public void shouldThrowNullPointerExceptionWhenActionIsMissing() {
+		try {
+			pagerSpliterator.tryAdvance(null);
+			Assert.fail("Missing NullPointerException");
+		} catch (Throwable e) {
+			assertEquals(NullPointerException.class, e.getClass());
+		}
+	}
+}
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