Prevent error in OptOutManager when language is not a string

[tsteur]

Had the other day close to 80 fatal errors from a scanner where language was not a string which resulted in an error below

preg_match(): Argument #2 ($subject) must be of type string, array given
Error: {"message":"preg_match(): Argument #2 ($subject) must be of type string, array given","file":"core/Filesystem.php","line":79,"request_id":"96ae7","backtrace":" on core/Filesystem.php(79)\n#0 core/Filesystem.php(79): preg_match('/(^[a-zA-Z0-9]+...', Array)\n#1 plugins/LanguagesManager/API.php(48): Piwik\Filesystem::isValidFilename(Array)\n#2 plugins/CoreAdminHome/OptOutManager.php(560):

Example URL: /index.php?action=optOut&backgroundColor=&fontColor=333333&fontFamily=Arial&fontSize=16px&language%5B0%5D=%3Dconvert%28int%2Csys.fn_sqlvarbasetostr%28HashBytes%28%27MD5%27%2C%271476824053%27%29%29%29and+3+in+&language%5B1%5D=3&module=CoreAdminHome

Review

• Functional review done
• Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• Wording review done
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• Documentation added if needed
• Existing documentation updated if needed

[bx80]

Recreated the error on 4.x-dev using test URL, no error with the test URL after this fix 👍

• Functional review done
• Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• n/a Wording review done
• n/a Code review done
• Not really useful Tests were added if useful/possible
• Reviewed for breaking changes
• not needed Developer changelog updated if needed
• not needed Documentation added if needed
• not needed Existing documentation updated if needed