in_kubernetes_events: allow setting kube_token_file as empty for use with kubectl proxy.

[pwhelan]

Summary

When using kubectl proxy it is possible to not use a token and skip authentication. This patch allows setting kube_token_file as an empty string to avoid having to load the token file.

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Testing
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• Example configuration file for the change
• Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

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• Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

Backporting

• Backport to latest stable release.

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