Fix management of non-string values in nxu:valueHolder tag

When using other kinds of values than strings on the nxu:valueHolder tag, it may misbehave even when using the attribute submitValue="false" as it will still render a hidden input with the value it holds.
When this value is not an instance of String, its string representation is used instead, and this might be an issue when this value is decoded after.

=> the UIValueHolder component needs to adapt some of its logics to avoid validation/rendering in such a case.