fix(instance): fix plugin segfault when bootscript is null

[utix]

Bootscript is deprecated cf https://www.scaleway.com/en/developers/api/instance/#path-instances-list-all-instances

seems relative to https://www.scaleway.com/en/docs/compute/instances/troubleshooting/bootscript-eol/

It appears that Bootscript is no longer available on API response

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.03%. Comparing base (fe86049) to head (4732203).
Report is 123 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2664      +/-   ##
==========================================
- Coverage   71.43%   71.03%   -0.40%     
==========================================
  Files         277      334      +57     
  Lines       35875    33475    -2400     
==========================================
- Hits        25628    23780    -1848     
+ Misses       8028     7388     -640     
- Partials     2219     2307      +88     

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