Possible issue when writing spatials that have lists as userdata

[riccardobl]

I wrote an unit test for this pr: #2093 and i've discovered a possible issue in the serialization of user data (that is unrelated to the PR).
You can reproduce it by setting testLists=true in the unit test:
https://github.com/jMonkeyEngine/jmonkeyengine/blob/master/jme3-plugins/src/test/java/com/jme3/export/JmeExporterTest.java#L139-L239
and then run ./gradlew test
It will fail with this log:

com.jme3.export.JmeExporterTest > testExporterConsistency[0] FAILED
    java.lang.AssertionError: Value does not match in map 1 for key int-list expected [1, 2, 3] but got null
        at org.junit.Assert.fail(Assert.java:89)
        at org.junit.Assert.assertTrue(Assert.java:42)
        at com.jme3.export.JmeExporterTest.compareMaps(JmeExporterTest.java:260)
        at com.jme3.export.JmeExporterTest.testExporterConsistency(JmeExporterTest.java:238)