Throw UOE for unsupported MultiPageSinkFactory.createSink(...) overloads

This prevent NPE which was previously experienced due to non-allowed
null value being returned.

This closes #1231

Author: kwin

src/main/java/org/apache/maven/plugins/site/render/ReportDocumentRenderer.java

@@ -146,21 +146,21 @@ public Sink createSink(File outputDirectory, String outputName) {
         }
 
         @Override
-        public Sink createSink(File arg0, String arg1, String arg2) throws IOException {
-            // Not used
-            return null;
+        public Sink createSink(File outputDir, String outputName, String encoding) throws IOException {
+            throw new UnsupportedOperationException(
+                    "Only createSink(File, String) is supported by MultiPageSinkFactory. The encoding is always determined by the site rendering context.");
         }
 
         @Override
-        public Sink createSink(OutputStream arg0) throws IOException {
-            // Not used
-            return null;
+        public Sink createSink(OutputStream out) throws IOException {
+            throw new UnsupportedOperationException(
+                    "Only createSink(File, String) is supported by MultiPageSinkFactory. OutputStream based sinks are not supported.");
         }
 
         @Override
-        public Sink createSink(OutputStream arg0, String arg1) throws IOException {
-            // Not used
-            return null;
+        public Sink createSink(OutputStream out, String encoding) throws IOException {
+            throw new UnsupportedOperationException(
+                    "Only createSink(File, String) is supported by MultiPageSinkFactory. OutputStream based sinks are not supported.");
         }
 
         public List<MultiPageSubSink> sinks() {