Update PHPStan level to 8

[westonruter]

This is a sub-PR of #1205. Review and merge it first.

See #775.

This fixes the following PHPStan issues (only 8!):

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  Line   includes/server-timing/load.php                                   
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  215    Anonymous function should return string but returns string|null.  
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  Line   plugins/optimization-detective/class-od-html-tag-walker.php                                        
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  229    Property OD_HTML_Tag_Walker::$open_stack_tags (array<string>) does not accept array<string|null>.  
  239    Generator expects value type string, string|null given.                                            
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  Line   plugins/webp-uploads/hooks.php                                                                
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  745    Parameter #1 $data of function wp_print_inline_script_tag expects string, string|null given.  
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  Line   plugins/webp-uploads/image-edit.php                                                                                                                                                                                                                                   
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  156    Offset 'sizes' might not exist on array{width: int, height: int, file: string, sizes: array<string, array{file: string, width: int, height: int, mime-type: string, sources?: array<string, array{file: string, filesize: int}>}>, image_meta: array<string, mixed>,  
         filesize: int, sources?: array<string, array{file: string, filesize: int}>, has_transparency?: bool, ...}|null.                                                                                                                                                       
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  Line   tests/plugins/optimization-detective/class-od-html-tag-walker-tests.php                                 
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  466    Method OD_HTML_Tag_Walker_Tests::export_array_snapshot() should return string but returns string|null.  
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  Line   tests/plugins/optimization-detective/optimization-tests.php                                         
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  1115   Anonymous function should return string but returns string|null.                                    
  1137   Method OD_Optimization_Tests::normalize_whitespace() should return string but returns string|null.  
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Please review individual commits for changes.

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Co-authored-by: westonruter <[email protected]>
Co-authored-by: swissspidy <[email protected]>
Co-authored-by: adamsilverstein <[email protected]>

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[westonruter]

The inclusion of this if statement wrapper is the only change to this block of code.

[adamsilverstein]

turning off whitespace confirms this

[adamsilverstein]

Spiffy!