[PATCH] oprofile: fix build on gcc-12

[Sergei T. <sl...@gm...>]

`gcc-12` will forbid std::string(nullptr_t) constructors
(as they never had a defined behaviour):

    operf_stats.cpp:182:24: error: use of deleted function 'std::__cxx11::basic_string...; std::nullptr_t = std::nullptr_t]'
      182 |                 return NULL;
          |                        ^~~~

Signed-off-by: Sergei Trofimovich <sl...@gm...>
---
 libperf_events/operf_stats.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libperf_events/operf_stats.cpp b/libperf_events/operf_stats.cpp
index 3cc28a75..a7bd0aec 100644
--- a/libperf_events/operf_stats.cpp
+++ b/libperf_events/operf_stats.cpp
@@ -179,7 +179,7 @@ static string create_stats_dir(string const & cur_sampledir)
 	if (rc && (errno != EEXIST)) {
 		cerr << "Error trying to create stats dir. " << endl;
 		perror("mkdir failed with");
-		return NULL;
+		return "";
 	}
 	return stats_dir;
 }
-- 
2.33.1