Fix default-repo by supporting nil as default value for flags

[briandealwis]

Fixes: #5588
Related: #5632, #5548

Description
Adds support for "nillable" values like the use of StringOrUndefined used for --default-repo.

Prior to #5632, we never initialized struct values (called Var in pflag). But with the new --port-forward implementation (#5554), we now needed to initialize struct values with possibly different values depending on the command (#5548).

With #5548 we now reset Var type values to their default value, but it turns out that --default-repo had an incorrect default value of the empty string:

skaffold/cmd/skaffold/app/cmd/flags.go

Lines 99 to 105 in bcb2eaa

	Name: "default-repo",
	Shorthand: "d",
	Usage: "Default repository value (overrides global config)",
	Value: &opts.DefaultRepo,
	DefValue: "",
	FlagAddMethod: "Var",
	DefinedOn: []string{"dev", "run", "debug", "deploy", "render", "build", "delete"},

This default value should have been nil: the default repo support differentiates between nil and the empty string "". When nil, Skaffold will look to see if there is a value set in the global config; whereas the empty string indicates that no default-repo should be used.

User facing changes (remove if N/A)

• the default-repo is able to be used from the global config again

[codecov]

Codecov Report

Merging #5654 (ec66a62) into master (bcb2eaa) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5654      +/-   ##
==========================================
+ Coverage   70.54%   70.57%   +0.02%     
==========================================
  Files         410      410              
  Lines       15639    15643       +4     
==========================================
+ Hits        11033    11040       +7     
+ Misses       3793     3792       -1     
+ Partials      813      811       -2     

Impacted Files	Coverage Δ
cmd/skaffold/app/cmd/flags.go	88.88% <100.00%> (+0.28%)	⬆️
pkg/skaffold/config/types.go	100.00% <100.00%> (ø)
pkg/skaffold/docker/image.go	79.53% <0.00%> (+1.39%)	⬆️

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[MarlonGamez]

This looks good to me. I've also tested locally to verify it works