Without this, the relative rarer types of requests seen in projects with
staging and handled by list command will be included in staging proposal.
However, since they are not stageable the select operation will fail. This
change ensures that a filter is always present when stageable is True to
exclude non-stableable requests. The list command sets stageable to false
in order to list out the non-stageable requests of interest.
This was not observed in openSUSE since the main non-stageable request was
change_devel and that was exluded in StrategyNone. That filter could be
replaced with the stageable filter, but having an always on filter seems to
make more sense since generally operating in one of two modes.
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