fix(notifier): don't panic on unconfigured notifier

[piksel]

When notifications is populated, but the corresponding options for configuring the notifications are not provided watchtower currently panics when trying to write out what notifications it uses.
This will instead simply print none, which is much more helpful.

Fixes #867

[codecov]

Codecov Report

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@@           Coverage Diff           @@
##             main     #869   +/-   ##
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  Coverage        ?   45.40%           
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  Files           ?       24           
  Lines           ?     1381           
  Branches        ?        0           
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  Hits            ?      627           
  Misses          ?      688           
  Partials        ?       66           

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