Use a cheaper is-empty test for a C-string

We only need to look at the first byte to determine whether the length
is > 0; no need to count how many more bytes after it aren't '\0'.

Change-Id: Id2db2a9a993086c0aee02c99396fcb346b1a348e
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/testlib/qtaptestlogger.cpp

@@ -157,7 +157,7 @@ void QTapTestLogger::addIncident(IncidentTypes type, const char *description,
     QTestCharBuffer directive;
     if (incident) {
         QTest::qt_asprintf(&directive, " # %s%s%s", incident,
-                           qstrlen(description) ? " " : "", description);
+                           description && description[0] ? " " : "", description);
     }
 
     int testNumber = QTestLog::totalCount();