[2833] use NULL instead of 0.

may need some more discussion, but not the point for this branch anyway,
and at least it's more consistent with rest of the source code.

src/lib/datasrc/tests/zone_loader_unittest.cc

@@ -292,7 +292,7 @@ protected:
     {
         // Use ROOT_NAME as a placeholder; it will be ignored if filename is
         // null.
-        prepareSource(Name::ROOT_NAME(), 0);
+        prepareSource(Name::ROOT_NAME(), NULL);
     }
     void prepareSource(const Name& zone, const char* filename) {
         // Cleanup the existing data in the right order