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.

Tests/Importer/ZaaImporter/LaurisItemTest.cs

@@ -86,6 +86,7 @@ namespace Tests.Importer.ZaaImporter
 
         [TestCase("Niedermol. Heparin (Anti-Xa): 0.99 U/ml;", "Niedermol. Heparin (Anti-Xa)", 0.99, "U/ml")]
         [TestCase("glomerul. Filtrationsr. CKD-EP: 42 ml/min /1,73qm;", "glomerul. Filtrationsr. CKD-EP", 42, "ml/min /1,73qm")]
+        [TestCase("NT-proBNP: 598 [s. Bem.] pg/ml;", "NT-proBNP", 598, "pg/ml")]
         public void ParseLaurisWithoutLimits(
             string laurisString, string name, double value,
             string unit)

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}",