FR #79344: xmlwriter_write_attribute_ns: $prefix should be nullable

FR #79344: xmlwriter_write_attribute_ns: $prefix should be nullable

The `$prefix` parameter of `xmlwriter_write_element_ns()` and
`xmlwriter_start_element_ns()` is nullable, what allows these functions
to be used instead of their non NS variants.  Consequently, we make the
`$prefix` parameter of `xmlwriter_write_attribute_ns()` and
`xmlwriter_start_attribute_ns()` nullable as well.