util/{Const,Writable}Buffer: eliminate static method Null(), use nullptr cast instead

src/decoder/plugins/ModplugDecoderPlugin.cxx

@@ -66,12 +66,12 @@ mod_loadfile(DecoderClient *client, InputStream &is)
 
 		if (size == 0) {
 			LogWarning(modplug_domain, "file is empty");
-			return { nullptr, 0 };
+			return nullptr;
 		}
 
 		if (size > MODPLUG_FILE_LIMIT) {
 			LogWarning(modplug_domain, "file too large");
-			return { nullptr, 0 };
+			return nullptr;
 		}
 
 		buffer.size = size;

src/util/AllocatedArray.hxx

@@ -72,7 +72,7 @@ public:
 
 	AllocatedArray(AllocatedArray &&other)
 		:buffer(other.buffer) {
-		other.buffer = Buffer::Null();
+		other.buffer = nullptr;
 	}
 
 	~AllocatedArray() {

src/util/ConstBuffer.hxx

@@ -60,10 +60,6 @@ struct ConstBuffer<void> {
 	constexpr ConstBuffer(pointer_type _data, size_type _size)
 		:data(_data), size(_size) {}
 
-	constexpr static ConstBuffer Null() {
-		return ConstBuffer(nullptr, 0);
-	}
-
 	constexpr static ConstBuffer<void> FromVoid(ConstBuffer<void> other) {
 		return other;
 	}
@@ -120,10 +116,6 @@ struct ConstBuffer {
 	constexpr ConstBuffer(const T (&_data)[_size])
 		:data(_data), size(_size) {}
 
-	constexpr static ConstBuffer Null() {
-		return ConstBuffer(nullptr, 0);
-	}
-
 	/**
 	 * Cast a ConstBuffer<void> to a ConstBuffer<T>, rounding down
 	 * to the next multiple of T's size.

src/util/WritableBuffer.hxx

@@ -60,10 +60,6 @@ struct WritableBuffer<void> {
 	constexpr WritableBuffer(pointer_type _data, size_type _size)
 		:data(_data), size(_size) {}
 
-	constexpr static WritableBuffer Null() {
-		return { nullptr, 0 };
-	}
-
 	constexpr bool IsNull() const {
 		return data == nullptr;
 	}
@@ -114,10 +110,6 @@ struct WritableBuffer {
 	constexpr WritableBuffer(T (&_data)[_size])
 		:data(_data), size(_size) {}
 
-	constexpr static WritableBuffer Null() {
-		return { nullptr, 0 };
-	}
-
 	/**
 	 * Cast a WritableBuffer<void> to a WritableBuffer<T>,
 	 * rounding down to the next multiple of T's size.