Remove zero size special case for copy_to_stream

This doesn't seem to serve a purpose anymore. Stats are expensive,
so doing an unnecessary stat just to short-circuit the zero size
c

Remove zero size special case for copy_to_stream

This doesn't seem to serve a purpose anymore. Stats are expensive,
so doing an unnecessary stat just to short-circuit the zero size
case is rather dubious. It can also break with stream wrappers
that return inaccurate sizes (symfony/symfony#40574) and probably
can also break with stream filters.

Drop the special case and adjust code to make it more obvious that
it will still be handled correctly.

Closes GH-6807.

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