There's a number of places in the WooCommerce codebase where the
built-in function 'round' is executed passing a non-numeric value
(not a number and not a string that can be parsed as a number),
for example round(''). In PHP 7 this yields a value of 0, but in
PHP 8 this throws an error.
This commit adds a 'NumberUtil' class with a static 'round' method,
this method checks if the passed value is numeric and if so it just
executes the built-in function, otherwise it returns 0. And all the
calls to 'round' in the codebase are replaced with 'NumberUtil::round'.
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