Allow InitialCredit/MoreCreditAfter of zero (#13067) (#13068)

#13046
introduced additional checks which prevent setting
`{credit_flow_default_credit,{0,0}}`.

Setting credits to zero allows disabling the credit flow mechanism
(we use it in our benchmarks and mention for example in
https://www.rabbitmq.com/blog/2023/03/21/native-mqtt)

(cherry picked from commit a4634d3)

Co-authored-by: Michal Kuratczyk <[email protected]>

Author: mergify[bot]

deps/rabbit/src/rabbit.erl

@@ -1761,8 +1761,8 @@ persist_static_configuration() ->
                      {ok, {InitialCredit, MoreCreditAfter}}
                        when is_integer(InitialCredit) andalso
                             is_integer(MoreCreditAfter) andalso
-                            InitialCredit > 0 andalso
-                            MoreCreditAfter > 0 andalso
+                            InitialCredit >= 0 andalso
+                            MoreCreditAfter >= 0 andalso
                             MoreCreditAfter =< InitialCredit ->
                          {InitialCredit, MoreCreditAfter};
                      Other ->