fix: Don't crash on non-existent valueFrom path values

[chihshenghuang]

What this PR does / why we need it:
This PR #1998 didn't fix the issue that kube-state-metrics will crash if the valueFrom path is non-exist. This issue is mentioned at #2141
How does this change affect the cardinality of KSM: (increases, decreases or does not change cardinality)
No
Which issue(s) this PR fixes:
Fixes #1992

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: chihshenghuang / name: Chih-Sheng Huang (4433379)

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[dashpole]

/assign @dgrisonnet
/triage accepted

[dgrisonnet]

This looks good to me.

@rexagod could you please also have a look?

[rexagod]

Right, my PR didn't fix it for fields that depended on path, such as valueFrom, but only the path itself. Only nit I'd suggest here is to reword the commit header and message since this doesn't seem to be a gauge-specific fix.

Thank you for this fix! 🙂

[chihshenghuang]

Thanks @rexagod for the review and comment! I updated the commit message, please take a look again!

[rexagod]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chihshenghuang, dgrisonnet, rexagod

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