Fix nil panic in memoryUsageCmd Lua script

[NilPuig]

Summary

MEMORY USAGE returns nil for Redis keys that no longer exist (e.g., expired or deleted task keys). In Lua, nil is converted to false (a boolean). The memoryUsageCmd script then attempts arithmetic on this boolean value, causing:

ERR user_script:30: attempt to perform arithmetic on local 'bytes' (a boolean value)

This breaks Inspector.GetQueueInfo() and the /api/queues endpoint in asynqmon, showing "Could not retrieve queues live data" in the UI.

Changes

• Add nil guards around all three MEMORY USAGE calls on task keys (lines 266, 283, 306)
• Add divide-by-zero guard on agg_task_sample_size for the aggregation group section

How to reproduce

This happens when task keys referenced in a queue's pending/active/scheduled/retry lists have expired or been deleted from Redis, but the list entries still reference them. MEMORY USAGE on a non-existent key returns nil, which becomes false in Lua.

Testing

Tested in production with Redis 7.2.4 and asynq v0.25.1 worker processing ~900K tasks/day. After applying this patch, the /api/queues endpoint returns data correctly.

Fixes #728
Related to #901

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.34%. Comparing base (4f00f52) to head (2fd155e).
⚠️ Report is 34 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1092      +/-   ##
==========================================
- Coverage   67.13%   63.34%   -3.80%     
==========================================
  Files          29       29              
  Lines        4300     4984     +684     
==========================================
+ Hits         2887     3157     +270     
- Misses       1135     1549     +414     
  Partials      278      278              

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