Improve handling of missing template values

[briandealwis]

Fixes: #6128

Description
Introduce new default function, modelled on sprig's default function:

 {{default "value" .MISSING}}

results in "value" if .MISSING is nil or a zero value.

[codecov]

Codecov Report

Merging #6136 (a3146db) into master (823896b) will increase coverage by 1.15%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master    #6136      +/-   ##
==========================================
+ Coverage   69.94%   71.10%   +1.15%     
==========================================
  Files         478      484       +6     
  Lines       18286    21617    +3331     
==========================================
+ Hits        12791    15370    +2579     
- Misses       4548     5264     +716     
- Partials      947      983      +36     

Impacted Files	Coverage Δ
pkg/skaffold/util/env_template.go	94.36% <87.50%> (-1.09%)	⬇️
pkg/skaffold/schema/latest/v2/config.go	5.40% <0.00%> (-94.60%)	⬇️
pkg/skaffold/schema/v3alpha1/config.go	2.70% <0.00%> (-47.30%)	⬇️
pkg/skaffold/render/generate/generate.go	52.38% <0.00%> (-24.98%)	⬇️
pkg/skaffold/initializer/build/builders.go	60.00% <0.00%> (-10.00%)	⬇️
pkg/skaffold/build/gcb/types.go	44.44% <0.00%> (-9.41%)	⬇️
cmd/skaffold/app/cmd/version.go	77.77% <0.00%> (-7.94%)	⬇️
pkg/skaffold/status/provider.go	75.00% <0.00%> (-6.82%)	⬇️
pkg/skaffold/build/cluster/types.go	93.33% <0.00%> (-6.67%)	⬇️
pkg/skaffold/sync/provider.go	65.00% <0.00%> (-6.43%)	⬇️
... and 471 more

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[briandealwis]

Holding off for input from @nkubala @viglesiasce

[nkubala]

#6128 (comment)

I'm fine with this, but ParseEnvTemplate() is also used when computing tags in the EnvTagger. I suppose this could change some behavior that users may have been relying on when their referenced values aren't set....but:

1. that's probably a very fringe corner case, and
2. this is probably the way we should have been doing this to begin with.

[tejal29]

Why not template.New("envTemplate").Option("missingkey=error").Parse(t) and error out when evaluating the template?

[briandealwis]

Why not template.New("envTemplate").Option("missingkey=error").Parse(t) and error out when evaluating the template?

I think the general expectation is that we should handle environment variables like the shell, which replaces non-existent variables with "".

$ echo hi${THIS_DOES_NOT_EXIST}there
hithere

[briandealwis]

That said, @tejal29's suggestion has some appeal. We could provide something like Helm's default function. That comes from the sprig template function library that Helm incorporates; that library adds more than a hundred functions.

Reworking as discussed in triage

[briandealwis]

I've reverted our template behaviour back to the original behaviour and introduced a new default function modelled on the sprig project's default.

[nkubala]

nice. I wonder how many other functions from https://masterminds.github.io/sprig/ would be useful to support here... 🤔

[nkubala]

are there other types we want to consider here? or another way of posing this question: how much of https://github.com/Masterminds/sprig/blob/master/defaults.go#L35-L60 should we reuse here?

[briandealwis]

Given that most of our uses will be environment strings, I don't expect any.