fix(library.js): change nulls in settings to empty strings

[ThorodanBrom]

• The initial settings seem to be passed to NodeBB at the very beginning, these settings are all null and are expected to be filled in at the Plugins -> OpenID Connect panel

• NodeBB v1.19.1 complains that the settings are null and refuses to start up; and the settings can never be filled in

• We replace the nulls with empty strings; this allows NodeBB to start and then the OpenID Connect panel can be filled out

• This issue from the NodeBB repo helped to get to the bottom of the problem

• The fix works on NodeBB 1.19.1. Was able to add an OIDC configuration and login.

• I'm not sure if empty strings are the correct or secure approach, please replace it with a better placeholder if possible

[ankeshanand1]

The current nodebb-plugin-fusionauth-oidc does not work with NodeBB v2.2.1 . Builds are successful but NodeBB crashes when a web request is made.

This commit fixes the problem! Verified it on NodeBB v2.2.1

[mooreds]

Thanks for the fix!