fix(config): Replace environment variables if existing but empty

[srebhan]

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

resolves #13565

Previous attempts (e.g. PR #13457) to restore the pre-1.27.0 environment-variable-replacement behavior missed one corner case where the environment variable exists but is empty. Versions before v1.27.0 will replace those variables (with an empty string) but the current code does not (see #13565).

This PR restores the old behavior by using the additional information about the application of the default replacement, indicating if an interpolation happened or if the variable could be mapped.

[orgads]

Is there a reason not to replace also non-existing vars with an empty string?

[telegraf-tiger]

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DEB	RPM	TAR GZ	ZIP
amd64.deb	aarch64.rpm	darwin_amd64.tar.gz	windows_amd64.zip
arm64.deb	armel.rpm	darwin_arm64.tar.gz	windows_arm64.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz	windows_i386.zip
armhf.deb	i386.rpm	freebsd_armv7.tar.gz
i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
mipsel.deb	s390x.rpm	linux_arm64.tar.gz
ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_riscv64.tar.gz
		linux_s390x.tar.gz

[orgads]

I can't think of a use-case that would expect a value to be "foo" if FOO=foo is set, and "${FOO}" if it is unset. Is this intentional?

[srebhan]

@orgads please think about e.g. #13460. The issue is that the pattern $... or ${...} is also used within config options that e.g. use go-templates or regular expressions. In any case, we want to keep the old (pre-1.27.0) behavior and this only replaces if the envvar exists.

[srebhan]

Depends on compose-spec/compose-go#431.

[powersj]

Feel free to land once unit tests pass