[Impeller] Don't crash if the context is unavailable.

Fixes flutter/flutter#123027.

Turns out, this isn't an issue with back-grounding. This can only happen if the
IO manager is torn down before worker pool has had a chance to process the
decompression job, or if the UI manager was never setup. It is hard to imaging
how this could happen in a real application. If it does though, we should not
crash. This patch makes the change to return an error instead. The linked issue
was a test environment and the failure was addressed in
flutter#40535.

Author: chinmaygarde

lib/ui/painting/image_decoder_impeller.cc

@@ -406,7 +406,10 @@ void ImageDecoderImpeller::Decode(fml::RefPtr<ImageDescriptor> descriptor,
        result,
        supports_wide_gamut = supports_wide_gamut_  //
   ]() {
-        FML_CHECK(context) << "No valid impeller context";
+        if (!context) {
+          result(nullptr);
+          return;
+        }
         auto max_size_supported =
             context->GetResourceAllocator()->GetMaxTextureSizeSupported();