Fix NPE when there was a parameter with only a listConverter

I was seeing the following NPE when there was a parameter with only a listConverter as seen in the updated JCommanderTest.testListAndSplitters. This pull request fixes that.

java.lang.NullPointerException at com.beust.jcommander.JCommander.instantiateConverter(JCommander.java:1320)
 at com.beust.jcommander.JCommander.convertValue(JCommander.java:1268) 
at com.beust.jcommander.JCommander.convertValue(JCommander.java:1212) 
at com.beust.jcommander.ParameterDescription.addValue(ParameterDescription.java:242) 
at com.beust.jcommander.ParameterDescription.addValue(ParameterDescription.java:210)
 at com.beust.jcommander.JCommander.processFixedArity(JCommander.java:868) 
at com.beust.jcommander.JCommander.processFixedArity(JCommander.java:850) 
at com.beust.jcommander.JCommander.parseValues(JCommander.java:722)
 at com.beust.jcommander.JCommander.parse(JCommander.java:282)
 at com.beust.jcommander.JCommander.parse(JCommander.java:265)
 at com.beust.jcommander.JCommanderTest.testListAndSplitters(JCommanderTest.java:587)