empty relgain and random db port

@karnem

Fix to use get random port with an input of range of ports and in case of relative gain not retrieved an empty constant would be used.

notebooks/pnCCD/Correct_pnCCD_NBC.ipynb

     metadata = ConstantMetaData()
     relgain = Constants.CCD(DetectorTypes.pnCCD).RelativeGain()
     metadata.calibration_constant = relgain
-
-    # set the operating condition
-    condition = Conditions.Illuminated.CCD(bias_voltage=bias_voltage,
-                                           integration_time=integration_time,
-                                           gain_setting=gain,
-                                           temperature=fix_temperature,
-                                           pixels_x=pixels_x,
-                                           pixels_y=pixels_y,
-                                           photon_energy=photon_energy)
-
-    device = Detectors.PnCCD1
-    metadata.detector_condition = condition
-
-
-    # specify the a version for this constant
-    metadata.calibration_constant_version = Versions.Now(device=device)
-    metadata.retrieve(cal_db_interface, timeout=cal_db_timeout)
-    constants["RelativeGain"] = relgain.data
-    print("Retrieved RelativeGain constant with creation time:"
-          f"{metadata.calibration_constant_version.begin_at}")
+    try:
+        # set the operating condition
+        condition = Conditions.Illuminated.CCD(bias_voltage=bias_voltage,
+                                               integration_time=integration_time,
+                                               gain_setting=gain,
+                                               temperature=fix_temperature,
+                                               pixels_x=pixels_x,
+                                               pixels_y=pixels_y,
+                                               photon_energy=photon_energy)
+
+        device = Detectors.PnCCD1
+        metadata.detector_condition = condition
+
+
+        # specify the a version for this constant
+        metadata.calibration_constant_version = Versions.Now(device=device)
+        cal_db_interface = get_random_db_interface(cal_db_interface)
+        metadata.retrieve(cal_db_interface, timeout=cal_db_timeout)
+        constants["RelativeGain"] = relgain.data
+        print("Retrieved RelativeGain constant with creation time:"
+              f"{metadata.calibration_constant_version.begin_at}")
+    except Exception as e:
+        print(f"Error: {e}.\n")
+        if relgain.data is None:
+            constants["RelativeGain"] = np.ones((pixels_x, pixels_y))
+            print("Relative was not retrieved and an empty constant of ones will be used.")
     display(Markdown('### Relative Gain Map Retrieval'))
     fig = xana.heatmapPlot(constants["RelativeGain"], figsize=(8, 8), x_label='Columns', y_label='Rows', lut_label='Relative Gain',
                            aspect=1, x_range=(0, pixels_y), y_range=(0, pixels_x), vmin=0.8, vmax=1.2,