by default make unresolved placeholder in scope of applyWithDeletion(…

…) return optional instead of the unresolved placeholder

Signed-off-by: Thomas Jaeckle <thomas.jaeckle@bosch-si.com>

model/placeholders/src/main/java/org/eclipse/ditto/model/placeholders/PlaceholderFilter.java

@@ -131,8 +131,8 @@ public static String apply(final String template, final ExpressionResolver expre
 
     /**
      * Finds all placeholders ({@code {{ ... }}}) defined in the given {@code template} and tries to replace them
-     * by applying the given {@code expressionResolver}. If a pipeline function deletes the element, then return
-     * an empty optional.
+     * by applying the given {@code expressionResolver}. If a pipeline function deletes the element or the pipeline
+     * leads to an unresolved element, then return an empty optional.
      *
      * @param template the template string.
      * @param resolver the expression-resolver used to resolve placeholders and optionally pipeline stages
@@ -147,7 +147,7 @@ public static Optional<String> applyWithDeletion(final String template, final Ex
         return resolver.resolve(template, true)
                 .accept(PipelineElement.<Optional<String>>newVisitorBuilder()
                         .resolved(Optional::of)
-                        .unresolved(() -> Optional.of(template))
+                        .unresolved(Optional::empty)
                         .deleted(Optional::empty)
                         .build());
     }