Fix crash if reference string is in fact not a reference.

E.g. NT-proBNP has a remark "[s. Bem.]" which represents neither
reference limits nor a categorical normal value.

- VERBESSERT: Kein Absturz mehr, wenn Labordaten NT-proBNP enthalten.

zaaReloaded2/Importer/ZaaImporter/LaurisItem.cs

@@ -154,6 +154,10 @@ namespace zaaReloaded2.Importer.ZaaImporter
                             LowerLimit = Double.Parse(limitMatch.Groups["limit2"].Value.Replace(',', '.'),
                                 CultureInfo.InvariantCulture);
                             break;
+                        case "":
+                            // NOOP for special cases such as "[s. Bem.]" which occurs in NT-proBNP
+                            // Fixes exception ID 65ca8575.
+                            break;
                         default:
                             throw new InvalidOperationException(
                                 String.Format("Unknown operator in {0}",