Add null check for the ServerCursor, as the server might not have created a cursor even if there are results.

 JAVA-1855

Author: jyemin

driver/src/main/com/mongodb/DBCursor.java

@@ -576,7 +576,7 @@ public DBCursor skip(final int numberOfElements) {
 
     @Override
     public long getCursorId() {
-        if (cursor != null) {
+        if (cursor != null && cursor.getServerCursor() != null) {
             return cursor.getServerCursor().getId();
         } else {
             return 0;

driver/src/test/functional/com/mongodb/DBCursorTest.java

@@ -210,6 +210,11 @@ public void testGetCursorId() {
         assertEquals(0, cursor.getCursorId());
         cursor.hasNext();
         assertThat(cursor.getCursorId(), is(not(0L)));
+
+        cursor = collection.find();
+        assertEquals(0, cursor.getCursorId());
+        cursor.hasNext();
+        assertThat(cursor.getCursorId(), is(0L));
     }
 
     @Test