Fix NullPointerException in TestPanel Prefs.getRecentMessageXmlFiles on fresh install

[jmwall]

Bug

On a fresh install (no existing prefs.xml), closing or removing a message from the TestPanel message list throws a NullPointerException at Prefs.java:388 because myRecentMessageXmlFiles is null when getRecentMessageXmlFiles() iterates over it.

Fix

Added a null check that initializes savedVals to an empty ArrayList<XmlFormat>() if myRecentMessageXmlFiles is null — matching the existing pattern already used for myOpenTableFiles in the same class.

Reproduction

1. Delete ~/HapiTestPanel/prefs.xml (or launch on a machine that has never run TestPanel)
2. Launch TestPanel
3. Open any message
4. Attempt to close/remove it
5. The dialog appears but clicking Yes/No/Cancel fails silently with an NPE in the console

Root Cause

Regression introduced in [c679d04](https://github.com/hapifhir/hapi-hl7v2/commit/c679d04c08e0d46c4d1036dd2408d80ac77a2347) ("Allow exporting and importing of profile group files", Sep 2012) which refactored Prefs.java from the Java Preferences API to JAXB-serialized XML fields.

The old code always produced a non-null list because ourPrefs.get() provided a default empty string:

// Old code — always safe, savedVals is never null
ArrayList<String> savedVals = new ArrayList<String>(
    Arrays.asList(ourPrefs.get(GET_RECENT_MESSAGE_FILES, "").split("\\n")));

The new code directly assigns the JAXB field, which is null until prefs.xml exists and has been deserialized:

// New code — null on fresh install when no prefs.xml exists
List<XmlFormat> savedVals = myRecentMessageXmlFiles;
for (XmlFormat string : savedVals) {  // NPE here

Notably, the same commit correctly handled the null case for the analogous myOpenTableFiles field:

if (myOpenTableFiles == null) {
    myOpenTableFiles = new ArrayList<String>();
}

This fix applies the same null-guard pattern that was used for myOpenTableFiles but was missed for myRecentMessageXmlFiles.