logging: fix logging empty threadname

src/logging.cpp

@@ -364,7 +364,8 @@ void BCLog::Logger::LogPrintStr(const std::string& str, const std::string& loggi
     }
 
     if (m_log_threadnames && m_started_new_line) {
-        str_prefixed.insert(0, "[" + util::ThreadGetInternalName() + "] ");
+        const auto threadname = util::ThreadGetInternalName();
+        str_prefixed.insert(0, "[" + (threadname.empty() ? "unknown" : threadname) + "] ");
     }
 
     str_prefixed = LogTimestampStr(str_prefixed);