Fix a linker error when using clang-12

When building the project with clang-12 we get an error
ld.lld: error: undefined symbol: bool std::operator==<re2::RE2>(std::shared_ptr<re2::RE2> const&, std::nullptr_t)
For some reason we cannot find the nullptr symbol.
That is why we change to using the NULL macro that should be OK in an if statement as we do not pass it to other functions

Author: landzhev

source/server/admin/stats_params.h

@@ -86,10 +86,10 @@ struct StatsParams {
       return false;
     }
 
-    if (re2_filter_ != nullptr && !re2::RE2::PartialMatch(metric.name(), *re2_filter_)) {
+    if (re2_filter_ != NULL && !re2::RE2::PartialMatch(metric.name(), *re2_filter_)) {
       return false;
     }
-
+    
     return true;
   }