Allow plugins to replace gerrit-provided hot-replaceable DynamicItem, too

While at it, clean up the code path for non hot-replaceable
DynamicItem (in particular adding a defensive check for null even
though it should never be triggered since a DynamicItem from gerrit
core cannot be unloaded).

Change-Id: I5f4074a3752e8d84fc349b502408bdd520927d5e
1 file changed