[tor-bugs] #20526 [Core Tor/Tor]: Implement `rend_service_is_ephemeral()`

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#20526: Implement `rend_service_is_ephemeral()`
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     Reporter:  twim          |      Owner:
         Type:  enhancement   |     Status:  new
     Priority:  Medium        |  Milestone:
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 Checking if directory is `NULL` is not so readable. Replace such checks
 with `rend_service_is_ephemeral()`.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/20526>
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