SQLConfig: Add Default value for connection string

SQLConfig/frmSQLConfig.vb

@@ -10,7 +10,7 @@ Public Class frmSQLConfig
     Private ReadOnly Logger As Logger
     Private ReadOnly LogConfig As LogConfig
 
-    Public Property ConnectionString As String
+    Public Property ConnectionString As String = String.Empty
 
     Public Sub New()
         ' Dieser Aufruf ist für den Designer erforderlich.