added Nullable for RelationalPath.getPrimaryKey()

querydsl-sql/src/main/java/com/mysema/query/sql/RelationalPath.java

@@ -7,6 +7,8 @@ package com.mysema.query.sql;
 
 import java.util.Collection;
 
+import javax.annotation.Nullable;
+
 import com.mysema.query.types.EntityPath;
 
 /**
@@ -22,6 +24,7 @@ public interface RelationalPath<T> extends EntityPath<T>{
      * 
      * @return
      */
+    @Nullable
     PrimaryKey<T> getPrimaryKey();
 
     /**