Werk #5460: Waiting without a wait condition immediately returns now

Note: This change only affects users writing their own tools for the Livestatus interface, and only those tools which rely on an edge case.

Up to now, Livestatus had a special obscure case: Using a WaitTrigger header in a query without a WaitCondition header resulted in waiting for the trigger exactly once. This is a misfeature, because it is effectively useless and doesn't do what one might expect: Due to spurious wakeups (https://en.wikipedia.org/wiki/Spurious_wakeup), waiting might return earlier basically at any time.

To fix this behaviour, the special case has been removed, and a missing WaitCondition always has its expected semantics (always true) now. Note that this is consistent with a missing Filter header, which means "no filtering".

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