fix(meta): prevent build failure for ios splash image when no icon is used

[maybebot]

When the icon is set to false in config, or when the icon is not found correctly, pwa._iosSplash is undefined.
This prevents access to its proprieties and thus the nuxt build failure.

I added also a console.warn that might help in debugging future issues, to try to make it obvious that no icons are actually being generated since the path to the base icon is wrong.

Fixes #355 and #334

[codecov]

Codecov Report

Merging #362 into master will decrease coverage by 0.23%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #362      +/-   ##
==========================================
- Coverage   85.40%   85.17%   -0.24%     
==========================================
  Files          10       10              
  Lines         370      371       +1     
  Branches      111      111              
==========================================
  Hits          316      316              
- Misses         50       51       +1     
  Partials        4        4              

Impacted Files	Coverage Δ
lib/icon/module.js	92.30% <0.00%> (-1.03%)	⬇️
lib/meta/module.js	81.90% <100.00%> (ø)

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[pi0]

Thanks!