Fixed bug that caused NullPointerException with custom training data

Added != null condition to gene comparison in MultiClassifier.java
Otherwise classification with own training sets resulted in a NullPointerException.
This was due to the unspecified -g option (which is not possible in conjunction with -t):
java -Xmx4000M -jar /software/RDPTools/classifier.jar classify --train_propfile=trained.properties --outputFile=out.txt --format=allrank --queryFile=query.fastq
Exception in thread "main" java.lang.NullPointerException
        at edu.msu.cme.rdp.multicompare.MultiClassifier.<init>(MultiClassifier.java:66)
        at edu.msu.cme.rdp.multicompare.MultiClassifier.<init>(MultiClassifier.java:72)
        at edu.msu.cme.rdp.multicompare.Main.main(Main.java:245)
        at edu.msu.cme.rdp.classifier.cli.ClassifierMain.main(ClassifierMain.java:65)
The additional condition should have no negative side effects.