OpenXML Filter: DOCX: a crash on merge occurs

When paragraphs are being merged and the base paragraph properties are absent, the following error can be observed:

java.lang.NullPointerException
	at net.sf.okapi.filters.openxml.Markup$General.lambda$apply$0(Markup.java:102)
	at java.util.ArrayList.forEach(ArrayList.java:1259)
	at net.sf.okapi.filters.openxml.Markup$General.apply(Markup.java:102)
	at net.sf.okapi.filters.openxml.Block$Markup.apply(Block.java:197)
	at net.sf.okapi.filters.openxml.StyledTextSkeletonWriter.processTextUnit(StyledTextSkeletonWriter.java:168)
	at net.sf.okapi.common.filterwriter.GenericFilterWriter.processTextUnit(GenericFilterWriter.java:259)
	at net.sf.okapi.common.filterwriter.GenericFilterWriter.handleEvent(GenericFilterWriter.java:195)
	at net.sf.okapi.filters.openxml.OpenXMLFilterWriter.handleEvent(OpenXMLFilterWriter.java:261)

For more details please refer to the attached document.

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