tests: benchmark: do not initialize the adhoc rule

Initializing the std::optional<std::string> adhoc rule variable to ""
will... insert a value into the optional! Removing the point of using an
optional and running the adhoc benchmark **every time**. Do not
initialize the optional.

tests/benchmark/benchmark.hpp

@@ -80,7 +80,7 @@ struct Config
     ::std::string srcdir = ".";
     ::std::string outfile = "results.json";
     ::std::string gitrev = "<unknown>";
-    ::std::optional<::std::string> adhoc = "";
+    ::std::optional<::std::string> adhoc;
     int adhocRepeat = 1;
     const ::std::string adhocBenchName = "bf_adhoc";
     int64_t gitdate = 0;