[Impeller] Don't crash in image decompression if the context is unavailable. (#40890)

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

lib/ui/painting/image_decoder_impeller.cc

diff --git a/lib/ui/painting/image_decoder_impeller.cc b/lib/ui/painting/image_decoder_impeller.cc
index 01ee5ce..4d70397 100644
--- a/lib/ui/painting/image_decoder_impeller.cc
+++ b/lib/ui/painting/image_decoder_impeller.cc
@@ -406,7 +406,10 @@
        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();