Some cleanups to initialize empty MetaData at fewer positions

[oscargus]

Fewer initialization of empty MetaData to simplify when moving to model.

[Siedlerchr]

@oscargus I get an NPE when starting JabRef:

Exception in thread "AWT-EventQueue-0" java.lang.ExceptionInInitializerError
    at net.sf.jabref.JabRefMain.start(JabRefMain.java:70)
    at net.sf.jabref.JabRefMain.lambda$0(JabRefMain.java:52)
    at java.awt.event.InvocationEvent.dispatch(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$500(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)
Caused by: java.lang.NullPointerException
    at net.sf.jabref.preferences.JabRefPreferences.getBooleanDefault(JabRefPreferences.java:986)
    at net.sf.jabref.preferences.JabRefPreferences.getBoolean(JabRefPreferences.java:978)
    at net.sf.jabref.model.entry.InternalBibtexFields.<clinit>(InternalBibtexFields.java:72)
    ... 16 more

[Siedlerchr]

Interesting. Now after a second run it seems to work correct...

[oscargus]

Strange. I do not have any problems starting. Looking at this PR, I strongly doubt that it affects anything since the new MetaData() is simply moved in two places, none which should be active during startup...

[oscargus]

Your error seems to relate to defaults in JabRefPreferences not being initialized when trying to access SpecialFieldsUtils.PREF_SERIALIZESPECIALFIELDS. I guess that is the risk of the static initializer in InternalBibtexFields, that other things may not have initialized yet. We should fix it so that it does not have a static initializer and gets the required preferences injected on startup from JabRefMain. Relates to JabRef#150