Ensure claim extractor does not attempt profile call when URL is empty

[JoelSpeed]

Description

This ensures that we don't attempt to make a profileURL call when the profileURL is empty.

Motivation and Context

Without this, the proxy logs an error when creating a session:

[2022/02/17 20:39:10] [oauthproxy.go:777] Error redeeming code during OAuth2 callback: could not get claim "groups": failed to fetch claims from profile URL: error making request to profile URL: error performing request: Get "": unsupported protocol scheme ""

CC @braunsonm as I know you found this while working on your PKCE PR

How Has This Been Tested?

Using the local testing environment, I could reproduce the issue until this fix was applied.

Checklist:

• My change requires a change to the documentation or CHANGELOG.
• I have updated the documentation/CHANGELOG accordingly.
• I have created a feature (non-master) branch for my PR.

[braunsonm]

Thanks!

[JoelSpeed]

Rebased to fix changelog conflicts, no other changes, will merge once the tests pass