Defined default implementation for the deprecated getConfigurationSource() method in order to remove it in the next major version

Author: urbim

common/src/main/java/com/kumuluz/ee/common/ConfigExtension.java

@@ -35,7 +35,9 @@ public interface ConfigExtension extends Extension {
      * @deprecated Use {@link #getConfigurationSources()} instead.
      */
     @Deprecated
-    ConfigurationSource getConfigurationSource();
+    default ConfigurationSource getConfigurationSource() {
+        return null;
+    }
 
     default List<ConfigurationSource> getConfigurationSources() {
         return Collections.emptyList();

core/src/main/java/com/kumuluz/ee/EeApplication.java

@@ -227,6 +227,7 @@ private void initialize() {
                 List<ConfigurationSource> sources = extension.getExtension().getConfigurationSources();
 
                 if (sources == null || sources.size() == 0) {
+                    //noinspection deprecation
                     sources = Collections.singletonList(extension.getExtension().getConfigurationSource());
                 }