[mlir][examples] Use {} instead of std::nullopt (NFC) (#146949)

This patch fixes deprecation warnings like:

  mlir/include/mlir/IR/TypeRange.h:51:19: error: 'ArrayRef' is
  deprecated: Use {} or ArrayRef<T>() instead
  [-Werror,-Wdeprecated-declarations]

Author: kazutakahirata

mlir/examples/toy/Ch2/mlir/MLIRGen.cpp

@@ -120,7 +120,7 @@ class MLIRGenImpl {
     // Arguments type are uniformly unranked tensors.
     llvm::SmallVector<mlir::Type, 4> argTypes(proto.getArgs().size(),
                                               getType(VarType{}));
-    auto funcType = builder.getFunctionType(argTypes, std::nullopt);
+    auto funcType = builder.getFunctionType(argTypes, {});
     return builder.create<mlir::toy::FuncOp>(location, proto.getName(),
                                              funcType);
   }

mlir/examples/toy/Ch3/mlir/MLIRGen.cpp

@@ -120,7 +120,7 @@ class MLIRGenImpl {
     // Arguments type are uniformly unranked tensors.
     llvm::SmallVector<mlir::Type, 4> argTypes(proto.getArgs().size(),
                                               getType(VarType{}));
-    auto funcType = builder.getFunctionType(argTypes, std::nullopt);
+    auto funcType = builder.getFunctionType(argTypes, /*results=*/{});
     return builder.create<mlir::toy::FuncOp>(location, proto.getName(),
                                              funcType);
   }

mlir/examples/toy/Ch4/mlir/MLIRGen.cpp

@@ -120,7 +120,7 @@ class MLIRGenImpl {
     // Arguments type are uniformly unranked tensors.
     llvm::SmallVector<mlir::Type, 4> argTypes(proto.getArgs().size(),
                                               getType(VarType{}));
-    auto funcType = builder.getFunctionType(argTypes, std::nullopt);
+    auto funcType = builder.getFunctionType(argTypes, /*results=*/{});
     return builder.create<mlir::toy::FuncOp>(location, proto.getName(),
                                              funcType);
   }

mlir/examples/toy/Ch5/mlir/MLIRGen.cpp

@@ -120,7 +120,7 @@ class MLIRGenImpl {
     // Arguments type are uniformly unranked tensors.
     llvm::SmallVector<mlir::Type, 4> argTypes(proto.getArgs().size(),
                                               getType(VarType{}));
-    auto funcType = builder.getFunctionType(argTypes, std::nullopt);
+    auto funcType = builder.getFunctionType(argTypes, /*results=*/{});
     return builder.create<mlir::toy::FuncOp>(location, proto.getName(),
                                              funcType);
   }

mlir/examples/toy/Ch6/mlir/MLIRGen.cpp

@@ -120,7 +120,7 @@ class MLIRGenImpl {
     // Arguments type are uniformly unranked tensors.
     llvm::SmallVector<mlir::Type, 4> argTypes(proto.getArgs().size(),
                                               getType(VarType{}));
-    auto funcType = builder.getFunctionType(argTypes, std::nullopt);
+    auto funcType = builder.getFunctionType(argTypes, /*results=*/{});
     return builder.create<mlir::toy::FuncOp>(location, proto.getName(),
                                              funcType);
   }

mlir/examples/toy/Ch7/mlir/MLIRGen.cpp

@@ -182,7 +182,7 @@ class MLIRGenImpl {
         return nullptr;
       argTypes.push_back(type);
     }
-    auto funcType = builder.getFunctionType(argTypes, std::nullopt);
+    auto funcType = builder.getFunctionType(argTypes, /*results=*/{});
     return builder.create<mlir::toy::FuncOp>(location, proto.getName(),
                                              funcType);
   }
@@ -441,10 +441,10 @@ class MLIRGenImpl {
     for (auto &var : lit.getValues()) {
       if (auto *number = llvm::dyn_cast<NumberExprAST>(var.get())) {
         attrElements.push_back(getConstantAttr(*number));
-        typeElements.push_back(getType(std::nullopt));
+        typeElements.push_back(getType(/*shape=*/{}));
       } else if (auto *lit = llvm::dyn_cast<LiteralExprAST>(var.get())) {
         attrElements.push_back(getConstantAttr(*lit));
-        typeElements.push_back(getType(std::nullopt));
+        typeElements.push_back(getType(/*shape=*/{}));
       } else {
         auto *structLit = llvm::cast<StructLiteralExprAST>(var.get());
         auto attrTypePair = getConstantAttr(*structLit);