Fix wp_tag_cloud() return type for PHPStan

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This PR fixes the following issue reported by PHPStan:

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  Line   admin/admin-filters-term.php                                          
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         Ignored error pattern #^Parameter \#3 \.\.\.\$values of function      
         sprintf expects bool\|float\|int\|string\|null,                       
         array<string>\|string\|void given\.$# in path                         
         /home/runner/work/polylang/polylang/admin/admin-filters-term.php was  
         not matched in reported errors.                                       
  482    Parameter #3 ...$values of function sprintf expects                   
         bool|float|int|string|null, array<string>|string given.               
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WTF PHPStan, how?!