Allow branding settings to be cleared in the UI

[knolleary]

Fixes #4831

The two branding settings branding:account:signUpTopBanner and branding:account:signUpLeftBanner can, for historic reasons, be set via the .yml or the admin UI.

If set in the yml, it can be overridden by the admin UI. However, if the admin UI sets it to a blank string, the value of yml is used again; there's no way for admin to override it to a blank string... until now.

Rather than just check the truthiness of these two settings, it now checks if they are not null (the default value).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.67%. Comparing base (4ab0fb2) to head (41ab061).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4841   +/-   ##
=======================================
  Coverage   78.67%   78.67%           
=======================================
  Files         314      314           
  Lines       15125    15125           
  Branches     3483     3483           
=======================================
  Hits        11900    11900           
  Misses       3225     3225           

Flag	Coverage Δ
backend	78.67% <100.00%> (ø)

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