Handle nil PortForward item on setting defaults

[Laterality]

Fixes: #5363

Description

Handles default-setting with list of PortForwards containing nil item.

User facing changes (remove if N/A)

When user run with following skaffold.yaml:

apiVersion: skaffold/v2beta10
kind: Config
metadata:
  name: bug
portForward:
 -

Then, will get looks like:

parsing skaffold config: setting default values: Config's portForward[0] is empty, Please check if it has valid values

[codecov]

Codecov Report

Merging #5416 (193cce0) into master (4b3c30f) will decrease coverage by 0.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5416      +/-   ##
==========================================
- Coverage   71.60%   71.37%   -0.23%     
==========================================
  Files         397      397              
  Lines       14434    14545     +111     
==========================================
+ Hits        10335    10382      +47     
- Misses       3333     3390      +57     
- Partials      766      773       +7     

Impacted Files	Coverage Δ
pkg/skaffold/schema/defaults/defaults.go	87.57% <100.00%> (+0.15%)	⬆️
pkg/skaffold/test/structure/structure.go	87.50% <0.00%> (-12.50%)	⬇️
pkg/skaffold/util/tar.go	50.66% <0.00%> (-5.34%)	⬇️
pkg/skaffold/runner/new.go	67.58% <0.00%> (-1.70%)	⬇️
pkg/skaffold/schema/versions.go	82.41% <0.00%> (ø)
pkg/skaffold/test/structure/error.go	100.00% <0.00%> (ø)
pkg/skaffold/test/error.go
pkg/skaffold/test/test.go
pkg/skaffold/test/structure/types.go
pkg/skaffold/schema/v2beta12/upgrade.go	100.00% <0.00%> (ø)
... and 3 more

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[MarlonGamez]

Thanks for opening this PR! Have a couple comments but this looks good :)

[MarlonGamez]

This looks good :) Just a small grammar change request

[MarlonGamez]

Thanks for making the changes :) Once CI passes I'll approve and merge!