We were hitting onImageStateChange with no texture active and hitting
the assert in that function for needing a texture.
There are times when we call onImageStateChange where there is no
texture bound, early exit in those cases.

Test:
  angle_deqp_khr_gles31_tests --user-angle=swiftshader --gtest_filter=dEQP.KHR_GLES31/core_layout_binding_image2D_layout_binding_imageLoad_*
  also:
  --gtest_filter=dEQP.KHR_GLES31/core_layout_binding_atomic_uint_layout_binding_atomic_ComputeShader
  --gtest_filter=dEQP.KHR_GLES31/core_layout_binding_buffer_layout_binding_atomicAdd_ComputeShader
  --gtest_filter=dEQP.KHR_GLES31/core_layout_binding_block_layout_binding_block_ComputeShader
  --gtest_filter=dEQP.KHR_GLES31/core_layout_binding_sampler2D_layout_binding_texture_ComputeShader

Bug: angleproject:4124
Change-Id: I027cf4ecb6a5d6aea874c306bf3e67b95a75af62
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1965027
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>