GET /api/overview: format cluster and node tag lists as maps

Empty proplists will be serialized to JSON as arrays,
which they arguably are, and HTTP API clients
expect a regardless of collection size.

References #12552 #12699

(cherry picked from commit 5c3ccc4)

deps/rabbitmq_management/src/rabbit_mgmt_wm_overview.erl

@@ -185,11 +185,13 @@ transform_retention_intervals([{MaxAgeInSeconds, _}|Rest], Acc) ->
     transform_retention_intervals(Rest, [AccVal|Acc]).
 
 cluster_tags() ->
-    case rabbit_runtime_parameters:value_global(cluster_tags) of
+    Val = case rabbit_runtime_parameters:value_global(cluster_tags) of
         not_found ->
             [];
         Tags -> Tags
-    end.
+    end,
+    rabbit_data_coercion:to_map(Val).
 
 node_tags() ->
-    application:get_env(rabbit, node_tags, []).
+    Val = application:get_env(rabbit, node_tags, []),
+    rabbit_data_coercion:to_map(Val).