[metrics-bugs] #23844 [Metrics/ExoneraTor]: Fix NullPointerException thrown by providing invalid parameters

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#23844: Fix NullPointerException thrown by providing invalid parameters
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     Reporter:  karsten             |      Owner:  metrics-team
         Type:  defect              |     Status:  new
     Priority:  Medium              |  Milestone:
    Component:  Metrics/ExoneraTor  |    Version:
     Severity:  Normal              |   Keywords:
Actual Points:                      |  Parent ID:
       Points:                      |   Reviewer:
      Sponsor:                      |
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 Looks like our recent changes from a few weeks ago introduced a
 `NullPointerException` when the user provides invalid paramters. For
 example,
 [https://exonerator.torproject.org/?ip=86.59.21.3k&timestamp=2017-10-13&lang=en
 this request for IP address 86.59.21.3k] throws such an exception. Should
 be relatively easy to resolve. Reported by ln5.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/23844>
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