NullPointerException in FilteredSortedSetDocValuesFacetCounts during query evaluation

Running the following query select [rep:facet(simple/tags)] from [nt:base] where contains([text], 'ipsum') with the following content

/content/foo
 - text = "lorem lorem"
 + simple/
   - tags = ["tag1", "tag2"]
/content/bar
 - text = "lorem ipsum"

runs in the following NPE

java.lang.NullPointerException
	at org.apache.jackrabbit.oak.plugins.index.lucene.util.FilteredSortedSetDocValuesFacetCounts.getTopChildren(FilteredSortedSetDocValuesFacetCounts.java:63)
	at org.apache.lucene.facet.MultiFacets.getTopChildren(MultiFacets.java:52)
	at org.apache.jackrabbit.oak.plugins.index.lucene.LucenePropertyIndex$LucenePathCursor$2.getValue(LucenePropertyIndex.java:1646)
	... 38 more

This is because the result set for the query only contains /content/bar and with that the count of the dimension simple/tag is 0. For that case SortedSetDocValuesFacetCounts#getDim() returns null and so does getTopChildren.

This expected behaviour is properly handled in LucenePropertyIndex.java#L1647 but not in FilteredSortedSetDocValuesFacetCounts.java#L63 where topChildren is dereferenced without null check.

To workaround that secure facets can be set to false, though the default value is true.