MPEG4Writer: Write a zero-length compressor string

The first byte is the number of bytes in the string to display,
the rest is the actual string (see ISO/IEC 14496-12:2008 page 29,
compressorname in VisualSampleEntry) to display. Currently, all 31
space characters are displayed in certain players (such as Apple
QuickTime Player) instead of the plain codec name (e.g. "H.264").
By writing the string length properly, the actual content of the
remaining 31 bytes is ignored instead of displayed.

Change-Id: I7f93a8966d74a4e86abc46c3dcb70f71780d9cd6