General: Make use of std::nullopt where applicable

Allows some implementations to avoid completely zeroing out the internal
buffer of the optional, and instead only set the validity byte within
the structure.

This also makes it consistent how we return empty optionals.

src/video_core/texture_cache/surface_base.cpp

@@ -115,20 +115,24 @@ std::optional<std::pair<u32, u32>> SurfaceBaseImpl::GetLayerMipmap(
     if (gpu_addr == candidate_gpu_addr) {
         return {{0, 0}};
     }
+
     if (candidate_gpu_addr < gpu_addr) {
-        return {};
+        return std::nullopt;
     }
+
     const auto relative_address{static_cast<GPUVAddr>(candidate_gpu_addr - gpu_addr)};
     const auto layer{static_cast<u32>(relative_address / layer_size)};
     if (layer >= params.depth) {
-        return {};
+        return std::nullopt;
     }
+
     const GPUVAddr mipmap_address = relative_address - layer_size * layer;
     const auto mipmap_it =
         Common::BinaryFind(mipmap_offsets.begin(), mipmap_offsets.end(), mipmap_address);
     if (mipmap_it == mipmap_offsets.end()) {
-        return {};
+        return std::nullopt;
     }
+
     const auto level{static_cast<u32>(std::distance(mipmap_offsets.begin(), mipmap_it))};
     return std::make_pair(layer, level);
 }