Don't detect file changes during hydration for virtual files

Don't detect file changes during hydration for virtual files

Indeed, that file size will almost always change between the 1 byte
placeholder and the hydrated file. Only when using the CfAPI on Windows
this won't be the case since because it will expose the original size
even for placeholders.

Also worth noting: the suffix backend didn't hit that case since the
filename changes (with suffix for placeholders, without for hydrated
files).

Signed-off-by: Kevin Ottens <kevin.ottens@nextcloud.com>