remove nil optional prefixes in dom_id_for
remove nil optional prefixes in dom_id_for

This allows a simpler logic when an optional dom id prefix
is passed into title_for_component:

  id = dom_id_for(component, options[:id_prefix], :title)

If options[:id_prefix] is nil then dom_id_for will process
this as if options[:id_prefix] is notthere at all.

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