fix: return error for empty Redis URL list

[dgivens]

When no Redis URLs are provided when creating a Redis session store, the parseRedisURLs() function returns a nil pointer, rather than an error, and causes a nil pointer dereference error

Description

Add a check for the length of []string input for parseRedisURLs() and returns an error if it's empty

Motivation and Context

Fixes #3100

I ran into this when I was spinning up the oauth2-proxy helm chart and misspelling the sessionStorage.redis.sentinel key as sessionStorage.redis.sentinels.

How Has This Been Tested?

Updated the unit tests. Built new image locally, deployed it into a local cluster, and verified that the correct error was returned.

Checklist:

• [ ] My change requires a change to the documentation or CHANGELOG.
• [ ] I have updated the documentation/CHANGELOG accordingly.
• [x] I have created a feature (non-master) branch for my PR.
• [x] I have written tests for my code changes.