[Tizen] Harden Vulkan external texture resolution

- Null-check the command buffer returned by CreateCommandBuffer()
  before dereferencing it in the Impeller path. It can be null when
  the context is being torn down.
- Fall back to the engine-provided size when the embedder does not
  populate the texture width/height, matching the Skia path.

Author: JSUYA

engine/src/flutter/shell/platform/embedder/embedder_external_texture_vulkan.cc

@@ -139,6 +139,11 @@ sk_sp<DlImage> EmbedderExternalTextureVulkan::ResolveTextureImpeller(
     return nullptr;
   }
 
+  if (texture_desc->width == 0 || texture_desc->height == 0) {
+    texture_desc->width = size.width();
+    texture_desc->height = size.height();
+  }
+
   auto& impeller_context =
       impeller::ContextVK::Cast(*aiks_context->GetContext());
 
@@ -153,6 +158,9 @@ sk_sp<DlImage> EmbedderExternalTextureVulkan::ResolveTextureImpeller(
   // Transition the layout to shader read.
   {
     auto buffer = impeller_context.CreateCommandBuffer();
+    if (!buffer) {
+      return nullptr;
+    }
     impeller::CommandBufferVK& buffer_vk =
         impeller::CommandBufferVK::Cast(*buffer);