fix(agent): Avoid panic by checking for skip_processors_after_aggregators

[srebhan]

Summary

Check for the option not being set and issue a warning just as we do for normal runs...

Checklist

• No AI generated code was used in this PR

Related issues

resolves #16399

[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

[Hipska]

I feel like this check should have been places somewhere else, because now the same code snippet now already exists 3 times in the codebase, and maybe probably another place to put it is forgotten.

Config.LoadAll would seem a good place to me.

I would also take into account the quiet CLI argument, and frankly the message itself doesn't make much sense in here, since you just want to check if a config file is valid or not. And wether you set this config item, the config is still valid.

[Hipska]

@srebhan