Fix LinalgOps build error on older clang versions

As reported in the PR llvm#123618, 0d4efa2 included a
construction of a `FailureOr` object with a `nullptr`, which didn't work
in at least clang-10.  This patch changes it into a constructor call
instead of a brace-init call so that it is unambiguous.

Author: erichkeane

mlir/lib/Dialect/Linalg/IR/LinalgOps.cpp

@@ -3533,7 +3533,8 @@ bool MatmulOp::isValidLhsRhsBroadcastMap(AffineMap bcastMap) {
 
 FailureOr<ArrayAttr> parseIndexingMapsAttr(OpAsmParser &parser) {
   if (parser.parseOptionalKeyword("indexing_maps"))
-    return {nullptr}; // Success in case indexing_maps was not provided.
+    return ArrayAttr{
+        nullptr}; // Success in case indexing_maps was not provided.
 
   ArrayAttr arrayAttr;
   if (parser.parseEqual() || parser.parseAttribute(arrayAttr))