dtoolutil: Fix compile warning in ExecutionEnvironment

dtool/src/dtoolutil/executionEnvironment.cxx

@@ -223,6 +223,8 @@ ns_has_environment_variable(const string &var) const {
 
 #ifndef PREREAD_ENVIRONMENT
   return getenv(var.c_str()) != nullptr;
+#else
+  return false;
 #endif
 }