NullPointerException validating a hybridization with no labeled extracts

[Nicklas Nordborg]

This is simple to reproduce:

1. Create a new Hybridization. Use the default settings. Make sure that no labeled extracts are selected.
2. Go to the "Overview" tab on the single-item view page of the new hybridization.
3. Click on the "Validate" button.

java.lang.NullPointerException
...at net.sf.basedb.util.overview.validator.LabeledExtractValidator.postValidateFolder(LabeledExtractValidator.java:122)
...at net.sf.basedb.util.overview.node.NodeFactory.postValidateFolder(NodeFactory.java:224)
...at net.sf.basedb.util.overview.loader.BasicItemNodeLoader.postValidateFolder(BasicItemNodeLoader.java:134)
...at net.sf.basedb.util.overview.loader.LabeledExtractLoader.createReverseNode(LabeledExtractLoader.java:230)
...at net.sf.basedb.util.overview.loader.LabeledExtractLoader.createReverseNode(LabeledExtractLoader.java:104)
...at net.sf.basedb.util.overview.loader.AutoChildNodeLoader.createReverseNode(AutoChildNodeLoader.java:95)
...at net.sf.basedb.util.overview.loader.HybridizationLoader.loadReverseChildNodes(HybridizationLoader.java:187)
...at net.sf.basedb.util.overview.loader.AbstractNodeLoader.loadChildNodesOfItemNode(AbstractNodeLoader.java:126)

...at net.sf.basedb.util.overview.loader.AbstractNodeLoader.loadChildNodes(AbstractNodeLoader.java:112)
...at net.sf.basedb.util.overview.loader.AutoChildNodeLoader.loadChildNodes(AutoChildNodeLoader.java:121)
...at net.sf.basedb.util.overview.GenericOverview.expand(GenericOverview.java:296)
...at org.apache.jsp.common.overview.index_jsp._jspService(index_jsp.java:115)

The error goes away as soon as at least one labeled extract is added to the hybridization.

[Nicklas Nordborg]

(In [5505]) Fixes #1553: NullPointerException validating a hybridization with no labeled extracts

Also fixes a follow-up issue so that the validator selects only one of MISSING_LABELED_EXTRACT or INCORRECT_NUM_LABELEDEXTRACTS when there are no labeled extracts.