hlo_instruction: Don't derefence a NULL OpMetadata during printing.

It's unclear as yet why this happens, but a NULL check is cheap to add.

PiperOrigin-RevId: 738911264

Author: pizzud

xla/hlo/ir/hlo_instruction.cc

@@ -3918,9 +3918,10 @@ void HloInstruction::PrintWithCanonicalNameMap(
   }
 
   if (options.print_metadata() &&
-      (!metadata_->op_type().empty() || !metadata_->op_name().empty() ||
-       !metadata_->source_file().empty() ||
-       !metadata_->scheduling_name().empty())) {
+      (metadata_ != nullptr &&
+       (!metadata_->op_type().empty() || !metadata_->op_name().empty() ||
+        !metadata_->source_file().empty() ||
+        !metadata_->scheduling_name().empty()))) {
     printer->Append(", metadata={");
     printer->Append(xla::OpMetadataToString(
         *metadata_, options.print_metadata_only_op_name()));