HTML API: Replace null-bytes in class_list class names

[sirreal]

Replace null bytes with the � replacement character in ::class_list.

The browser will replace null bytes with � in attribute values:

U+0000 NULL
This is an unexpected-null-character parse error. Append a U+FFFD REPLACEMENT CHARACTER character to the current attribute's value.

If we consider that class_list is a processed version of the class attribute, this null-byte replacement is appropriate. At this time, get_attribute is closer to "raw" access to the attribute value as authored.

Related to #7184.

Trac ticket: https://core.trac.wordpress.org/ticket/61531

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[dmsnell]

@sirreal I think we decided earlier that we can't rely on short forms of the default class. @costdev do you remember discussing the use of @covers ::class_list vs. @covers WP_HTML_Tag_Processor::class_list when the @coversDefaultClass is set? I thought you changed some code before to expand them.

[dmsnell]

Merged in [58969]
b37cbf9

[sirreal]

I'm pretty confident that with @coversDefaultClass correctly set, then @covers ::methodName works fine. There are existing examples of that in the codebase.

[dmsnell]

Absolutely @sirreal - but I'm speaking more about WPCS conventions than working code. I know there are examples in the codebase already demonstrating :: without the class and that it works fine. I just want to make sure we don't introduce code that is going to upset anyone.