[mlir][examples] Fix deprecation warnings for ArrayRef(std::nullopt)

Author: kuhar

mlir/examples/toy/Ch2/include/toy/Ops.td

@@ -297,7 +297,7 @@ def ReturnOp : Toy_Op<"return", [Pure, HasParent<"FuncOp">,
 
   // Allow building a ReturnOp with no return operand.
   let builders = [
-    OpBuilder<(ins), [{ build($_builder, $_state, std::nullopt); }]>
+    OpBuilder<(ins), [{ build($_builder, $_state, {}); }]>
   ];
 
   // Provide extra utility definitions on the c++ operation class definition.

mlir/examples/toy/Ch3/include/toy/Ops.td

@@ -298,7 +298,7 @@ def ReturnOp : Toy_Op<"return", [Pure, HasParent<"FuncOp">,
 
   // Allow building a ReturnOp with no return operand.
   let builders = [
-    OpBuilder<(ins), [{ build($_builder, $_state, std::nullopt); }]>
+    OpBuilder<(ins), [{ build($_builder, $_state, {}); }]>
   ];
 
   // Provide extra utility definitions on the c++ operation class definition.

mlir/examples/toy/Ch4/include/toy/Ops.td

@@ -335,7 +335,7 @@ def ReturnOp : Toy_Op<"return", [Pure, HasParent<"FuncOp">,
 
   // Allow building a ReturnOp with no return operand.
   let builders = [
-    OpBuilder<(ins), [{ build($_builder, $_state, std::nullopt); }]>
+    OpBuilder<(ins), [{ build($_builder, $_state, {}); }]>
   ];
 
   // Provide extra utility definitions on the c++ operation class definition.

mlir/examples/toy/Ch5/include/toy/Ops.td

@@ -335,7 +335,7 @@ def ReturnOp : Toy_Op<"return", [Pure, HasParent<"FuncOp">,
 
   // Allow building a ReturnOp with no return operand.
   let builders = [
-    OpBuilder<(ins), [{ build($_builder, $_state, std::nullopt); }]>
+    OpBuilder<(ins), [{ build($_builder, $_state, {}); }]>
   ];
 
   // Provide extra utility definitions on the c++ operation class definition.

mlir/examples/toy/Ch6/include/toy/Ops.td

@@ -335,7 +335,7 @@ def ReturnOp : Toy_Op<"return", [Pure, HasParent<"FuncOp">,
 
   // Allow building a ReturnOp with no return operand.
   let builders = [
-    OpBuilder<(ins), [{ build($_builder, $_state, std::nullopt); }]>
+    OpBuilder<(ins), [{ build($_builder, $_state, {}); }]>
   ];
 
   // Provide extra utility definitions on the c++ operation class definition.

mlir/examples/toy/Ch7/include/toy/Ops.td

@@ -360,7 +360,7 @@ def ReturnOp : Toy_Op<"return", [Pure, HasParent<"FuncOp">,
 
   // Allow building a ReturnOp with no return operand.
   let builders = [
-    OpBuilder<(ins), [{ build($_builder, $_state, std::nullopt); }]>
+    OpBuilder<(ins), [{ build($_builder, $_state, {}); }]>
   ];
 
   // Provide extra utility definitions on the c++ operation class definition.