[MLIR][NFC] Make AsmPrinter messages on null structures consistent

[bondhugula]

Summary

Make AsmPrinter messages on null structures consistent: use <<NULL ...>>.

mlir/lib/IR/AsmPrinter.cpp

 //===- AsmPrinter.cpp - MLIR Assembly Printer Implementation --------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //

 }
 
 void IntegerSet::dump() const {
   print(llvm::errs());
   llvm::errs() << "\n";
 }
 
 void AffineExpr::print(raw_ostream &os) const {
-  if (expr == nullptr) {
-    os << "null affine expr";
+  if (!expr) {
+    os << "<<NULL AFFINE EXPR>>";
     return;
   }
   ModulePrinter(os).printAffineExpr(*this);
 }
 
 void AffineExpr::dump() const {
   print(llvm::errs());
   llvm::errs() << "\n";
 }
 
 void AffineMap::print(raw_ostream &os) const {
-  if (map == nullptr) {
-    os << "null affine map";
+  if (!map) {
+    os << "<<NULL AFFINE MAP>>";
     return;
   }
   ModulePrinter(os).printAffineMap(*this);
 }
 
 void IntegerSet::print(raw_ostream &os) const {
   ModulePrinter(os).printIntegerSet(*this);
 }