[2834] use NULL instead of 0

based on discussion in #2833

src/lib/datasrc/tests/zone_finder_context_unittest.cc

@@ -72,7 +72,7 @@ typedef DataSourceClientPtr (*ClientCreator)(RRClass, const Name&);
 DataSourceClientPtr
 createInMemoryClient(RRClass zclass, const Name& zname) {
     const internal::CacheConfig cache_conf(
-        "MasterFiles", 0, *Element::fromJSON(
+        "MasterFiles", NULL, *Element::fromJSON(
             "{\"cache-enable\": true,"
             " \"params\":"
             "  {\"" + zname.toText() + "\": \"" +

src/lib/datasrc/tests/zone_loader_unittest.cc

@@ -312,7 +312,7 @@ protected:
                 string(TEST_DATA_DIR) + "/" + filename + "\"";
         }
         const internal::CacheConfig cache_conf(
-            "MasterFiles", 0, *Element::fromJSON(
+            "MasterFiles", NULL, *Element::fromJSON(
                 "{\"cache-enable\": true,"
                 " \"params\": {" + param_data + "}}"), true);
         ztable_segment_.reset(memory::ZoneTableSegment::create(