Make Image field optional for LMT

[rammanoj]

What this PR does / why we need it:

• Make Image field optional for LMT

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

[codecov]

Codecov Report

❌ Patch coverage is 9.09091% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.87%. Comparing base (d57486e) to head (2a318e4).

Files with missing lines	Patch %	Lines
internal/webhook/v1alpha2/linodemachine_webhook.go	9.09%	8 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #896      +/-   ##
==========================================
- Coverage   65.96%   65.87%   -0.09%     
==========================================
  Files          71       71              
  Lines        7956     7966      +10     
==========================================
  Hits         5248     5248              
- Misses       2406     2414       +8     
- Partials      302      304       +2     

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