fix(inputs.zfs): Avoid panic by handling explicitly empty kstat metrics

[srebhan]

Summary

Setting the kstatMetrics option explicitly to an empty string (i.e. kstatMetrics = [""]) prevents the default collection of those metrics (as the list is not empty) which might be desirable in some cases. However, the current code uses the entries literally for filenames and will panic.

This PR handles empty entries, i.e. explicitly disabling kstat metrics, correctly and moves code to an Init function to improve readability.

Checklist

• No AI generated code was used in this PR
• AI generated code used in this PR follows the InfluxData Policy on AI-Generated Code Contributions

Related issues

resolves #17952

[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

[skartikey]

@srebhan Clean fix using Init() to properly validate and filter empty kstatMetrics, preventing the panic while maintaining backward compatibility.