call ping to set connection for avoiding error (take place of #7215)

[uuip]

fix an error which is introduced by pr7040.

Description

when subscribed_to is empty , call ping to set connection attr .
After this operation, drain_events exec self._pubsub.get_message is ok,
and redis parse_response no error .

[lgtm-com]

This pull request fixes 1 alert when merging 217a956 into 9377e94 - view on LGTM.com

fixed alerts:

• 1 for Unused import

[codecov]

Codecov Report

Merging #7220 (217a956) into master (0620eb2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #7220   +/-   ##
=======================================
  Coverage   89.31%   89.32%           
=======================================
  Files         138      138           
  Lines       16769    16770    +1     
  Branches     2450     2450           
=======================================
+ Hits        14977    14979    +2     
+ Misses       1560     1559    -1     
  Partials      232      232           

Flag	Coverage Δ
unittests	89.31% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
celery/backends/redis.py	92.22% <100.00%> (+0.02%)	⬆️
celery/backends/asynchronous.py	69.26% <0.00%> (+0.43%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9377e94...217a956. Read the comment docs.

[auvipy]

looks good to me