Fix to StringValue in Json

If the argument is not a JsonString, it calls toString on it instead of
returning the empty string.

Json/src/ca/uqac/lif/cep/json/StringValue.java

@@ -1,6 +1,6 @@
 /*
     BeepBeep, an event stream processor
-    Copyright (C) 2008-2023 Sylvain Hallé
+    Copyright (C) 2008-2024 Sylvain Hallé
 
     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU Lesser General Public License as published
@@ -41,6 +41,6 @@ public String getValue(JsonElement x)
     {
       return ((JsonString) x).stringValue();
     }
-    return "";
+    return x.toString();
   }
 }