fix: avoid eager default interpolation

[seekskyworld]

Issue

• [BUG] False "variable is not set" warning when using the ${A:+} interpolation syntax docker/compose#12637

Summary

• defer default/error message interpolation until the branch is selected
• add tests ensuring unused defaults do not log warnings

Motivation

• avoid false "variable is not set" warnings for ${A:+...} and ${A?…} when the nested branch is not evaluated

Validation

• go test ./template