Fix a bug when set memo in pinot visibility store

[neil-xie]

What changed?
Currently it exists memo not equals to nil but data is empty, pinot will save it to search attribute and cause issue when list workflow executions and deserialize

Why?
Bug fix

How did you test it?
Unit test

Potential risks

Release notes

Documentation Changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.31%. Comparing base (bf088f2) to head (d77430d).

Additional details and impacted files

Files	Coverage Δ
common/persistence/pinot/pinot_visibility_store.go	93.23% <100.00%> (ø)

... and 6 files with indirect coverage changes

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[coveralls]

Pull Request Test Coverage Report for Build 018f30bb-dcf0-4fbe-a31a-33230e9129c8

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 63 unchanged lines in 15 files lost coverage.
• Overall coverage decreased (-0.02%) to 67.915%

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Files with Coverage Reduction	New Missed Lines	%
common/task/weighted_round_robin_task_scheduler.go	2	89.05%
common/persistence/sql/sqlplugin/postgres/task.go	2	73.4%
common/task/parallel_task_processor.go	2	93.06%
common/persistence/execution_manager.go	2	83.74%
service/matching/db.go	2	73.23%
service/matching/matcher.go	2	90.72%
common/log/tag/tags.go	3	50.46%
service/history/task/fetcher.go	3	85.57%
service/history/queue/timer_queue_processor_base.go	4	77.66%
service/frontend/wrappers/metered/metered.go	4	66.67%

Totals
Change from base Build 018f2f7f-db2f-46d8-b64d-f304962b54b6:	-0.02%
Covered Lines:	99721
Relevant Lines:	146831

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💛 - Coveralls