NPE in rare cases in Command.canReduce, invoked by XML export

[bengtmartensson]

If the commands to be exported starts with a signal without a certain parameter (except F), and a later command, with the same protocol contains that parameter, a NPE will occur.

For example NEC1 D=123,F=... followed by NEC1 D=123,S=45,F=... will trigger the problem.

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException: Cannot invoke "java.lang.Long.equals(Object)" because the return value of "java.util.Map.get(Object)" is null
	at org.harctoolbox.girr.Command.canReduce(Command.java:1278)
	at org.harctoolbox.girr.Command.toElement(Command.java:1173)
	at org.harctoolbox.girr.CommandSet.lambda$toElement$4(CommandSet.java:317)
	at java.base/java.util.LinkedHashMap$LinkedValues.forEach(LinkedHashMap.java:647)
	at org.harctoolbox.girr.CommandSet.toElement(CommandSet.java:316)
	at org.harctoolbox.girr.Remote.toElement(Remote.java:280)
	at org.harctoolbox.girr.RemoteSet.toElement(RemoteSet.java:552)
	at org.harctoolbox.girr.XmlExporter.toDocument(XmlExporter.java:204)```