Validator erweitert

Shared/Core/GkvValidator.cs

@@ -202,6 +202,9 @@ namespace BS.Shared.Core
         }
 		public static bool IsInvoiceNumberValid(string invoiceNumber)
 		{
+            if (string.IsNullOrEmpty(invoiceNumber))
+                return false;
+
 			var pattern = @"^(?!.*[-/]{2})(?![-/])(?!.*[-/]$)[A-Za-z0-9]+(?:[/-][A-Za-z0-9]+)*$";
 
             return Regex.IsMatch(invoiceNumber, pattern);