PHP8.1 compatibility fixes in referrers plugin

[MichaelRoosz]

With Matomo 4.9.0, I see lots of these warnings:

1.

WARNING Referrers
plugins/Referrers/Columns/Base.php(525): 
Deprecated - mb_strtolower(): Passing null to parameter #1 ($string) of type string is deprecated

2.

WARNING Referrers
plugins/Referrers/Columns/Base.php(478): 
Deprecated - urldecode(): Passing null to parameter #1 ($string) of type string is deprecated

Problem 1 I solved temporarily by replacing
$this->keywordReferrerAnalyzed = mb_strtolower($this->keywordReferrerAnalyzed);
with
$this->keywordReferrerAnalyzed = mb_strtolower($this->keywordReferrerAnalyzed ?? '');

But I am not sure this is the best fix.

[justinvelluppillai]

Hi @MichaelRoosz - that fix looks fine to me, it'd be great if you could make a Pull Request for that. The other warning would be a similar fix to the same file.

[MichaelRoosz]

Hello @justinvelluppillai , here is a pr: #19135