Make coinbaseaux flags optional

Needed for solo mining on bitcoind, starting with v0.20.0
It's probably safe to delete the coinbase flags, but leave them in as optional for now.

See also: bitcoin/bitcoin@9aedabe

cgminer.c

@@ -2960,7 +2960,7 @@ static bool gbt_solo_decode(struct pool *pool, json_t *res_val)
 	flags = json_string_value(json_object_get(coinbase_aux, "flags"));
 	default_witness_commitment = json_string_value(json_object_get(res_val, "default_witness_commitment"));
 
-	if (!previousblockhash || !target || !version || !curtime || !bits || !coinbase_aux || !flags) {
+	if (!previousblockhash || !target || !version || !curtime || !bits) {
 		applog(LOG_ERR, "Pool %d JSON failed to decode GBT", pool->pool_no);
 		return false;
 	}
@@ -3039,10 +3039,12 @@ static bool gbt_solo_decode(struct pool *pool, json_t *res_val)
 	ofs += ser_number(pool->scriptsig_base + ofs, height); // max 5
 
 	/* Followed by flags */
-	len = strlen(flags) / 2;
-	pool->scriptsig_base[ofs++] = len;
-	hex2bin(pool->scriptsig_base + ofs, flags, len);
-	ofs += len;
+	if (flags) {
+		len = strlen(flags) / 2;
+		pool->scriptsig_base[ofs++] = len;
+		hex2bin(pool->scriptsig_base + ofs, flags, len);
+		ofs += len;
+	}
 
 	/* Followed by timestamp */
 	cgtime(&now);