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/memory_manager.cpp

@@ -58,7 +58,7 @@ void MemoryManager::Unmap(GPUVAddr gpu_addr, std::size_t size) {
 std::optional<GPUVAddr> MemoryManager::AllocateFixed(GPUVAddr gpu_addr, std::size_t size) {
     for (u64 offset{}; offset < size; offset += page_size) {
         if (!GetPageEntry(gpu_addr + offset).IsUnmapped()) {
-            return {};
+            return std::nullopt;
         }
     }
 
@@ -135,13 +135,13 @@ std::optional<GPUVAddr> MemoryManager::FindFreeRange(std::size_t size, std::size
         }
     }
 
-    return {};
+    return std::nullopt;
 }
 
 std::optional<VAddr> MemoryManager::GpuToCpuAddress(GPUVAddr gpu_addr) const {
     const auto page_entry{GetPageEntry(gpu_addr)};
     if (!page_entry.IsValid()) {
-        return {};
+        return std::nullopt;
     }
 
     return page_entry.ToAddress() + (gpu_addr & page_mask);