Unify meaning of routerset NULL and routerset "" to avoid recurrence of #31495

From #31611 (moved):

Avoid configuration types where NULL and "" mean different things. We would have to add a new routerset type meaning "all routers", (maybe, "*"?) and have that be the default for EntryNodes. [We probably could not make NULL mean the same as "" for all cases, since there are lots of string-valued configuration types.]

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