EGL_KHR_no_config_context allows creating a context with
EGL_NO_CONFIG. Add a null check when collecting config
attributes. This fixes trace capture of Angry Birds 2 by
allowing CaptureReplayTest to pass EGL_DONT_CARE
for the config parameters.

Bug: angleproject:4817
Change-Id: I982136ae2a7db4bd8d3f16f4accfe093e5610091
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2333356
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>