Fix occasional crash when downloading assets from the Asset Library

This is caused by GitHub not publishing a Content-Length header in
all cases (it only does so if the file was requested recently),
which in turn made `String.humanize_size()` try to humanize a size of
-1 byte (as returned by HTTPRequest when no Content-Length
is contained in the response).
This crashed the editor due to a division by zero.

This closes #21200.

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