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/core/hle/service/nvdrv/devices/nvhost_as_gpu.cpp

@@ -265,7 +265,7 @@ std::optional<nvhost_as_gpu::BufferMap> nvhost_as_gpu::FindBufferMap(GPUVAddr gp
         }
     }
 
-    return {};
+    return std::nullopt;
 }
 
 void nvhost_as_gpu::AddBufferMap(GPUVAddr gpu_addr, std::size_t size, VAddr cpu_addr,
@@ -286,7 +286,7 @@ std::optional<std::size_t> nvhost_as_gpu::RemoveBufferMap(GPUVAddr gpu_addr) {
         return size;
     }
 
-    return {};
+    return std::nullopt;
 }
 
 } // namespace Service::Nvidia::Devices

src/core/hle/service/nvdrv/nvdrv.h

@@ -54,7 +54,7 @@ struct EventInterface {
             }
             mask = mask >> 1;
         }
-        return {};
+        return std::nullopt;
     }
     void SetEventStatus(const u32 event_id, EventState new_status) {
         EventState old_status = status[event_id];

src/core/hle/service/nvflinger/nvflinger.cpp

@@ -114,7 +114,7 @@ std::optional<u64> NVFlinger::OpenDisplay(std::string_view name) {
                      [&](const VI::Display& display) { return display.GetName() == name; });
 
     if (itr == displays.end()) {
-        return {};
+        return std::nullopt;
     }
 
     return itr->GetID();
@@ -124,7 +124,7 @@ std::optional<u64> NVFlinger::CreateLayer(u64 display_id) {
     auto* const display = FindDisplay(display_id);
 
     if (display == nullptr) {
-        return {};
+        return std::nullopt;
     }
 
     const u64 layer_id = next_layer_id++;
@@ -144,7 +144,7 @@ std::optional<u32> NVFlinger::FindBufferQueueId(u64 display_id, u64 layer_id) co
     const auto* const layer = FindLayer(display_id, layer_id);
 
     if (layer == nullptr) {
-        return {};
+        return std::nullopt;
     }
 
     return layer->GetBufferQueue().GetId();

src/core/hle/service/sockets/bsd.cpp

@@ -497,7 +497,7 @@ std::pair<s32, Errno> BSD::PollImpl(std::vector<u8>& write_buffer, std::vector<u
             return {0, Errno::SUCCESS};
         }
 
-        std::optional<FileDescriptor>& descriptor = file_descriptors[pollfd.fd];
+        const std::optional<FileDescriptor>& descriptor = file_descriptors[pollfd.fd];
         if (!descriptor) {
             LOG_ERROR(Service, "File descriptor handle={} is not allocated", pollfd.fd);
             pollfd.revents = POLL_NVAL;