Detector: make calibration_file optional

(cherry picked from commit 08f6457e)

src/core/detector/Detector.cpp

@@ -112,7 +112,9 @@ Detector::Detector(const Configuration& config) : m_role(DetectorRole::NONE) {
         LOG(TRACE) << "  Time resolution: " << Units::display(m_timeResolution, {"ms", "us"});
     }
 
+    if(config.has("calibration_file")) {
         m_calibrationfile = config.getPath("calibration_file");
+    }
 
     if(!isAuxiliary()) {
         if(config.has("mask_file")) {