Fix PHP 8.1 deprecation errors

[wtommyw]

Re-enabled deprecation errors in tests and fixed them.

Main fixes:

• CStarRating widget throwing a deprecation error if it is initalized without value;
• strcmp, strtolower, mb_stringtolower, mb_strpos and other built in functions being passed null in tests;
• CFileValidatorTest testing for the wrong value, it returns 0 as the outcome is always cased to an int. The test never failed because assertEquals uses == and performs type coercion;
• CStringValidator treating null as an empty string, strlen(null) is deprecated so a cast is needed;
• ClientScript tests using a reference to an uninitialised variable, for example: https://github.com/yiisoft/yii/blob/1c6f6fe221404de9afeb32059e6671676237482c/tests/framework/web/widgets/CAutoCompleteTest.php#L31 causing a deprecation error in: https://github.com/yiisoft/yii/blob/1c6f6fe221404de9afeb32059e6671676237482c/framework/web/CClientScript.php#L414 as $output would be null.

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌
Tests pass?	✔️
Fixed issues	#4506

[wtommyw]

@marcovtwout @rob006 I tried to request a re-review on both of you, but it looks like Github does not allowe this. I'm unsure if anyon's got the ping now

[marcovtwout]

@wtommyw After https://github.com/yiisoft/yii/pull/4503/files#r1055466147, feel free to add a changelog line with something like : "Enh 4503: Update tests/code for PHP 8.1 compatibility and stop suppressing deprecation warnings"

[wtommyw]

I still think that cast in getCanonicalID is unnecessary and null should be handled in getLocaleDisplayName() https://github.com/yiisoft/yii/pull/4503/files#r1055466147

@rob006 I agree, I've made the change. This way we'll actually follow the documented behaviour (not passing null into getCanonicalID.