Add null default backend environment variable option ANGLE_DEFAULT_PLATFORM environment variable now accepts 'null' as a valid option. This will set the default platform type to the null backend. Bug: angleproject:3893 Change-Id: Ieddcdf3ae840f927c1b276e56f3b753b67d9b73d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1784062 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Mohan Maiya <m.maiya@samsung.com> Commit-Queue: Mohan Maiya <m.maiya@samsung.com>