Check for for null collection elements in POJO CollectionCodec before deferring to codec.encode.

This fixes an NPE when a POJO has a nested collection element and one of the nested collection values is null.
JAVA-2756

Author: robrua

bson/src/main/org/bson/codecs/pojo/CollectionPropertyCodecProvider.java

@@ -53,7 +53,11 @@ private static class CollectionCodec<T> implements Codec<Collection<T>> {
         public void encode(final BsonWriter writer, final Collection<T> collection, final EncoderContext encoderContext) {
             writer.writeStartArray();
             for (final T value : collection) {
-                codec.encode(writer, value, encoderContext);
+                if (value == null) {
+                    writer.writeNull();
+                } else {
+                    codec.encode(writer, value, encoderContext);
+                }
             }
             writer.writeEndArray();
         }