Template arguments should not be null

If the generated annotation isn't available use the empty string instead of
null.

Fixes #645

RELNOTES=N/A

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=202048570

Author: cushon

value/src/main/java/com/google/auto/value/processor/AutoAnnotationProcessor.java

@@ -182,7 +182,7 @@ private void processMethod(ExecutableElement method) {
   private String getGeneratedTypeName() {
     return generatedAnnotation(elementUtils, processingEnv.getSourceVersion())
         .map(generatedAnnotation -> TypeEncoder.encode(generatedAnnotation.asType()))
-        .orElse(null);
+        .orElse("");
   }
 
   /**