NPE in TrinidadSkinProvider when base skin is not included in skin def in trinidad-skins.xml

In our application, we encountered this NPE, whenever base skin is not mentioned in the skin definition in trinidad-skins.xml (trivial to build a test-case thus). This needs to be fixed, because Trinidad framework is supposed to assume the default base skin based on renderkit or otherwise default to Trinidad's "simple.desktop" skin.

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1. java.lang.NullPointerException
2. at
   org.apache.myfaces.trinidadinternal.skin.provider.TrinidadSkinProvider.loadAvailableSkin(TrinidadSkinProvider.java:163)
3. at
   org.apache.myfaces.trinidadinternal.skin.provider.BaseSkinProvider._getMatchingSkin(BaseSkinProvider.java:216)
4. at
   org.apache.myfaces.trinidadinternal.skin.provider.BaseSkinProvider.getSkin(BaseSkinProvider.java:60)
5. at
   org.apache.myfaces.trinidadinternal.skin.provider.SkinProviderRegistry.getSkin(SkinProviderRegistry.java:116)
6. at
   org.apache.myfaces.trinidadinternal.renderkit.core.CoreRenderingContext._initializeSkin(CoreRenderingContext.java:635)
7. at
   org.apache.myfaces.trinidadinternal.renderkit.core.CoreRenderingContext.<init>(CoreRenderingContext.java:89)