fix: fix determine_redis_provider test behaviour (#6176)

Unit tests fails when a different REDIS_URL is defined since the provider
string `p` could be an empty string. This causes all RedisConnections to
use 127.0.0.1:6379 as the connection parameters.

Co-authored-by: Sylvester Chin <schin@gitlab.com>

lib/sidekiq/redis_connection.rb

@@ -66,9 +66,7 @@ def determine_redis_provider
           EOM
         end
 
-        ENV[
-          p || "REDIS_URL"
-        ]
+        ENV[p.to_s] || ENV["REDIS_URL"]
       end
     end
   end