tu: pCounterBuffers can be NULL in vkCmd*TransformFeedbackEXT()

tu: pCounterBuffers can be NULL in vkCmd*TransformFeedbackEXT()

According to the spec:

"pCounterBuffers is an optional array of buffer handles [...]
 If pCounterBuffers is NULL, then transform feedback will start
 capturing vertex data to byte offset zero in all bound transform
 feedback buffers."

"If counterBufferCount is not 0, and pCounterBuffers is not NULL,
 pCounterBuffers must be a valid pointer to an array [...]"

So counterBufferCount could be non-zero with pCounterBuffers
being NULL.

Fixes crash in RenderDoc when inspecting draw call with tesselation
or geometry shader present.

Fixes: 98b0d900 "turnip: rework streamout state and add missing counter buffer read/writes"
Signed-off-by: Danylo Piliaiev <dpiliaiev@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8140>