psbt: really hacky patch over libwally's 0-input txs parse fail

Issue being tracked at https://github.com/ElementsProject/libwally-core/pull/273

bitcoin/psbt.c

@@ -688,6 +688,11 @@ struct wally_psbt *psbt_from_b64(const tal_t *ctx,
 	else
 		psbt = NULL;
 	tal_wally_end(tal_steal(ctx, psbt));
+
+	/* FIXME: Patch for the empty-tx bug in libwally */
+	if (!psbt && strlen(str) == 28)
+		psbt = create_psbt(ctx, 0, 0, 0);
+
 	return psbt;
 }