fix(inputs.snmp): Avoid sending a nil to gosmi's GetEnumBitsFormatted

[powersj]

Summary

There is an upstream PR to resolve an issue when a nil value is passed. However that PR has not been looked at or merged. As such, this attempts to catch that scenario in Telegraf first.

See: sleepinggenius2/gosmi#45

Checklist

• No AI generated code was used in this PR

Related issues

fixes: #15200

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_arm64.tar.gz, and windows_amd64.zip.
Downloads for additional architectures and packages are available below.

👍 This pull request doesn't change the Telegraf binary size

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Artifact URLs

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]

Looks okay to me

[powersj]

@llamafilm have you had a chance to confirm this fix?

[llamafilm]

No. I'm still trying to figure out how to reproduce the crash reliably, before I try this new build.
Some other issues are getting in the way, like the one I posted on slack yesterday.

[srebhan]

@llamafilm please let us know if you were able to test. I would like to include this in our next release which is due on September 9th, so it would be good to have your say before...