Handle empty tracing property case

Test: set berberis.tracing to '' make sure it does not crash
Bug: http://b/232598137
Change-Id: Ia5104ed3e66b13e0bfcdf0d9b153e09da6dec5ca
diff --git a/base/tracing.cc b/base/tracing.cc
index be34629..5954178 100644
--- a/base/tracing.cc
+++ b/base/tracing.cc
@@ -120,7 +120,7 @@
   ScopedErrno scoped_errno;
 
   auto env = GetTracingConfig();
-  if (!env) {
+  if (!env || !env[0]) {
     return;
   }