Call writeAtomicCounterBufferDriverUniformOffsets method
only if there are valid atomic counter buffers

Bug: angleproject:3566
Change-Id: I03c3fd2095e30381597bfc66acae0ccbf828d401
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2383787
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: back sept 10 - Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>