ElementLink: return false in isValid if lookup fails
Change isValid to return false when called on an ElementLink with an
invalid index (e.g. beyond the end). This means that the behavior is the
same as if the target container was entirely missing.
This use-case comes up in the Trigger EDM deserialization where we try to restore missing containers during reconstruction (e.g. because they were forgotten to be serialized during data-taking, see !65540 (comment 7078959)). However, there might be ElementLinks pointing to these (now empty) containers.