According to the OpenGL ES 3.0 spec
(section 2.9.2 Creating Buffer Object Data Stores)

"If data is NULL, then the contents of the buffer
 object’s data store are undefined."

So the function should behave the same way, perform
the same checks, but simply skip the copy.

Change-Id: If49e37a8e836618389e105b5377ff183ac3e3107
Reviewed-on: https://swiftshader-review.googlesource.com/3601Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>