fix: generate sizes when only a single sizes is provided

[Baroshem]

related #645

[netlify]

👷 Deploy request for nuxt-image pending review.

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Name	Link
🔨 Latest commit	2f80b2f

[danielroe]

Would you check the failing CI? It might just be a matter of updating snapshots.

[Baroshem]

@danielroe

I tried to work on this Pr right now but I recently changed laptop to a newer one where I am experiencing some issues with the sharp library which blocks me from further working on this.

[error] 
Something went wrong installing the "sharp" module

Cannot find module '../build/Release/sharp-darwin-arm64v8.node'
Require stack:
- /image/node_modules/.pnpm/sharp@0.32.1/node_modules/sharp/lib/sharp.js
- /image/node_modules/.pnpm/sharp@0.32.1/node_modules/sharp/lib/constructor.js
- /image/node_modules/.pnpm/sharp@0.32.1/node_modules/sharp/lib/index.js

Possible solutions:
- Install with verbose logging and look for errors: "npm install --ignore-scripts=false --foreground-scripts --verbose sharp"
- Install for the current darwin-arm64v8 runtime: "npm install --platform=darwin --arch=arm64v8 sharp"
- Consult the installation documentation: https://sharp.pixelplumbing.com/install

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (576aaf3) 89.18% compared to head (da999d4) 89.18%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #920   +/-   ##
=======================================
  Coverage   89.18%   89.18%           
=======================================
  Files          44       44           
  Lines        2904     2904           
  Branches      322      321    -1     
=======================================
  Hits         2590     2590           
  Misses        313      313           
  Partials        1        1           

Files Changed	Coverage Δ
src/runtime/image.ts	86.13% <100.00%> (ø)

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