[mlir:PDL] Fix assembly format for pdl.apply_native_rewrite

[rriddle]

Summary

The results of a rewrite are optional, but we currently require
them to be present in the assembly format. This commit
makes the results component in the format optional.

mlir/include/mlir/Dialect/PDL/IR/PDLOps.td

let description = [{
     ```
   }];
 
   let arguments = (ins StrAttr:$name,
                        Variadic<PDL_AnyType>:$args,
                        OptionalAttr<ArrayAttr>:$constParams);
   let results = (outs Variadic<PDL_AnyType>:$results);
   let assemblyFormat = [{
-    $name ($constParams^)? (`(` $args^ `:` type($args) `)`)? `:` type($results)
-    attr-dict
+    $name ($constParams^)? (`(` $args^ `:` type($args) `)`)?
+    (`:` type($results)^)? attr-dict
   }];
 }
 
 //===----------------------------------------------------------------------===//
 // pdl::AttributeOp
 //===----------------------------------------------------------------------===//
 
 def PDL_AttributeOp : PDL_Op<"attribute"> {

mlir/test/Dialect/PDL/ops.mlir

 pdl.pattern @infer_type_from_type_used_in_match : benefit(1) {
   %types = pdl.types
   %root = pdl.operation -> (%types : !pdl.range<type>)
   pdl.rewrite %root {
     %otherTypes = pdl.types : [i32, i64]
     %newOp = pdl.operation "foo.op" -> (%types, %otherTypes : !pdl.range<type>, !pdl.range<type>)
   }
 }
+
+// -----
+
+pdl.pattern @apply_rewrite_with_no_results : benefit(1) {
+  %root = pdl.operation
+  pdl.rewrite %root {
+    pdl.apply_native_rewrite "NativeRewrite"(%root : !pdl.operation)
+  }
+}