Device created datetime isn't always provided

[frenck]

Proposed Changes

Reported in, home-assistant/core#61599.
Apparently the created value isn't always provided.

[codecov-commenter]

Codecov Report

Merging #26 (f7d53ec) into main (db532a9) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #26   +/-   ##
=======================================
  Coverage   94.00%   94.00%           
=======================================
  Files           4        4           
  Lines         100      100           
  Branches       14       14           
=======================================
  Hits           94       94           
  Misses          5        5           
  Partials        1        1           

Impacted Files	Coverage Δ
src/tailscale/models.py	97.72% <100.00%> (ø)

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