[#YANGTOOLS-990] NormalizedNodeStreamWriterMetadataDecorator pushes nulls into Deque

NormalizedNodeStreamWriterMetadataDecorator is pushing nulls into ArrayDeque, leading to NPE:

Caused by: java.lang.NullPointerException
	at java.util.ArrayDeque.addFirst(ArrayDeque.java:233) ~[?:?]
	at java.util.ArrayDeque.push(ArrayDeque.java:508) ~[?:?]
	at org.opendaylight.yangtools.rfc7952.data.util.NormalizedNodeStreamWriterMetadataDecorator.enterMetadataNode(NormalizedNodeStreamWriterMetadataDecorator.java:147) ~[?:?]
	at org.opendaylight.yangtools.rfc7952.data.util.NormalizedNodeStreamWriterMetadataDecorator.startMapNode(NormalizedNodeStreamWriterMetadataDecorator.java:96) ~[?:?]
	at org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeWriter.wasProcessedAsCompositeNode(NormalizedNodeWriter.java:217) ~[?:?]
	at org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeWriter.write(NormalizedNodeWriter.java:97) ~[?:?]
	at org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeWriter.writeChildren(NormalizedNodeWriter.java:164) ~[?:?]
	at org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeWriter.wasProcessedAsCompositeNode(NormalizedNodeWriter.java:180) ~[?:?]
	at org.opendaylight.yangtools.yang.data.api.schema.stream.NormalizedNodeWriter.write(NormalizedNodeWriter.java:97) ~[?:?]
	at org.opendaylight.yangtools.rfc7952.data.util.NormalizedMetadataWriter.write(NormalizedMetadataWriter.java:113) ~[?:?]

[YANGTOOLS-990] NormalizedNodeStreamWriterMetadataDecorator pushes nulls into Deque Created: 16/May/19 Updated: 16/May/19 Resolved: 16/May/19
Status:	Resolved
Project:	yangtools
Component/s:	data-impl
Affects Version/s:	3.0.0
Fix Version/s:	3.0.1

[YANGTOOLS-990] NormalizedNodeStreamWriterMetadataDecorator pushes nulls into Deque Created: 16/May/19 Updated: 16/May/19 Resolved: 16/May/19