truncating /dev/null to zero is always successful

Calling truncate on /dev/null can happen by the download methods if they
are instructed to download a file to /dev/null (as testcases are only
interested in the status code, but do not support HEAD requests yet)

So just ignore truncate calls on the /dev/null file as it is always
empty anyway, so truncating to zero isn't a problem.

Git-Dch: Ignore

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