incorrect behaviour of cert_aia_caissuers if file does not exists

[rtib]

In the case of deferred evaluation of cert_aia_caissuers, when the argument does not point to an existing and readable file, the function will fail and stop the agent.

This adds an appropriate test case and fix for this issue.

[puppet-community-rangefinder]

cert_aia_caissuers is a function

that may have no external impact to Forge modules.

This module is declared in 2 of 576 indexed public Puppetfiles.

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