Fix jira_connect_proxy_url blank setting bug

What does this MR do and why?

This fixes a bug where Jira shows an error message when installing the GitLab.com for Jira Cloud app in development mode with an blank jira_connect_proxy_url setting.

Screenshots or screen recordings

How to set up and validate locally

1. Start a Gitpod to get a publicly available GitLab instance.
2. Open up the 3000 port.
3. On the GitLab instance, go to /admin/application_settings/general#js-jira-connect-application-id-settings and select Save without changing the settings. This will result in the jira_connect_proxy_url application setting being an empty string instead of nil.
4. Follow the guide to set up a Jira app development environment.
5. Installing the app should fail with The app host returned HTTP response code 401 when we tried to contact it during installation..
6. Switch to the andysoiron/fix-jira-connect-proxy-url-blank-setting-bug branch.
7. Try again.
8. The installation should now be successful.

MR acceptance checklist

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