ConfigurationSource#getProperty(propertyName, defaultValue) throws NullPointerException when defaultValue is null

The reason is that CommonsConfigurationSource calls devaultValue.getClass() which fails when defaultValue is null. Stacktrace:

Caused by: java.lang.NullPointerException
  at org.xwiki.configuration.internal.CommonsConfigurationSource.getProperty(CommonsConfigurationSource.java:60)
  at org.xwiki.configuration.internal.CompositeConfigurationSource.getProperty(CompositeConfigurationSource.java:117)

The javadoc of ConfigurationSource#getProperty(propertyName, defaultValue) doesn't specify that the defaultValue must not be null. We should either fix the javadoc or the implementation.