[mlir] Use std::nullopt instead of None in comments (NFC)

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

mlir/lib/Dialect/SparseTensor/Transforms/SparseTensorCodegen.cpp

@@ -115,7 +115,7 @@ static scf::ForOp createFor(OpBuilder &builder, Location loc, Value upper,
 }
 
 /// Gets the dimension size for the given sparse tensor at the given
-/// original dimension 'dim'. Returns None if no sparse encoding is
+/// original dimension 'dim'. Returns std::nullopt if no sparse encoding is
 /// attached to the given tensor type.
 static Optional<Value> sizeFromTensorAtDim(OpBuilder &builder, Location loc,
                                            RankedTensorType tensorTp,