Address PHPStan warnings in 'classes/urlhelper.php'.

Intentionally skipping the line 66 one for now; adding an 'is_array' check clears the warning, but there's a larger topic of how to handle an invalid '' that doesn't result in an array.

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Line   classes/urlhelper.php
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66     Offset 'path' on array{scheme: string} in isset() does not exist.
165    Parameter #2 $associative of function get_headers expects bool, int
        given.
167    Parameter #2 $associative of function get_headers expects bool, int
        given.
278    Negated boolean expression is always true.
309    Negated boolean expression is always true.
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