Fix UB with guest-less Texture/Buffers in MarkGpuDirty

As there was no check for the lack of a `GuestTexture`/`GuestBuffer`, it would lead to UB when a texture/buffer that had no guest such as the `zeroTexture` from `GraphicsContext` would be marked as dirty they would cause a call to `NCE::RetrapRegions` with a `nullptr` handle that would be dereferenced and cause a segmentation fault.

app/src/main/cpp/skyline/gpu/buffer.cpp

@@ -45,7 +45,7 @@ namespace skyline::gpu {
     }
 
     void Buffer::MarkGpuDirty() {
-        if (dirtyState == DirtyState::GpuDirty || externallySynchronized) {
+        if (dirtyState == DirtyState::GpuDirty || externallySynchronized || !guest) {
             externallySynchronized = false; // We want to handle synchronize internally after the GPU work is done
             return;
         }