TT#165251 redis: don't fail on fist check of connection

* we are checking that the variable is not nil but on
  start we were setting the value to an empty metatable

Change-Id: Ib5b409b5c6e837baef38c35c7e619a46f44d51b9
(cherry picked from commit 3a008159b2)
(cherry picked from commit a1a7bc3da4)
(cherry picked from commit eba8c0130d)

ngcp/dlgcnt.lua

@@ -57,8 +57,6 @@ end
                 check_pair_dup = false,
                 allow_negative = false
             },
-            central = {},
-            pair = {}
         };
         return t;
     end

ngcp/dlglist.lua

@@ -56,8 +56,6 @@ end
                 },
                 check_pair_dup = false
             },
-            central = {},
-            pair = {}
         };
         return t;
     end

ngcp/recentcalls.lua

@@ -50,7 +50,6 @@ end
                 expire = 7200,
                 out_expire = 86400
             },
-            central = {},
         };
         return t;
     end