Allow missing keys in jsonpath

[smarterclayton]

It is common in constrained circumstances to prefer an empty string
result from JSONPath templates for missing keys over an error. Several
other implementations provide this (the canonical JS and PHP, as well as
the Java implementation). This also mirrors gotemplate, which allows
Options("missingkey=zero").

Added simple check and simple test case.

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This change is

[k8s-bot]

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[k8s-bot]

Can one of the admins verify that this patch is reasonable to test? If so, please reply "ok to test".
(Note: "add to whitelist" is no longer supported. Please update configurations in kubernetes/test-infra/jenkins/job-configs/kubernetes-jenkins-pull instead.)

This message will repeat several times in short succession due to jenkinsci/ghprb-plugin#292. Sorry.

[smarterclayton]

ok to test

[brendandburns]

LGTM

[k8s-bot]

GCE e2e build/test passed for commit bcea2c8.

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[k8s-github-robot]

@k8s-bot test this [submit-queue is verifying that this PR is safe to merge]

[k8s-bot]

GCE e2e build/test passed for commit bcea2c8.

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[k8s-github-robot]

Automatic merge from submit-queue