[4.x] Fix URL override example in TenancyServiceProvider stub

[lukinovec]

This PR fixes the URL override example in TenancyServiceProvider stub (the commented overrideUrlInTenantContext() segment). If the tenant doesn't have any domain, set the root URL back to the original one.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.92%. Comparing base (e3701f1) to head (500905f).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1426   +/-   ##
=========================================
  Coverage     85.92%   85.92%           
  Complexity     1147     1147           
=========================================
  Files           184      184           
  Lines          3360     3360           
=========================================
  Hits           2887     2887           
  Misses          473      473           

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