SLING-8116 - ValueMap - implement default methods using OSGI Converter

* added a private constructor for the ObjectConverter class
diff --git a/src/main/java/org/apache/sling/api/wrappers/impl/ObjectConverter.java b/src/main/java/org/apache/sling/api/wrappers/impl/ObjectConverter.java
index 066acd3..69c1dac 100644
--- a/src/main/java/org/apache/sling/api/wrappers/impl/ObjectConverter.java
+++ b/src/main/java/org/apache/sling/api/wrappers/impl/ObjectConverter.java
@@ -53,7 +53,9 @@
                 .rule(new TypeRule<ZonedDateTime, Calendar>(ZonedDateTime.class, Calendar.class, ObjectConverter::toCalendar))
                 .rule(new TypeRule<ZonedDateTime, String>(ZonedDateTime.class, String.class, ObjectConverter::toString))
                 .build();
-}
+    }
+
+    private ObjectConverter() {}
 
     private static String toString(ZonedDateTime zonedDateTime) {
         return DateTimeFormatter.ISO_OFFSET_DATE_TIME.format(zonedDateTime);