Fix saving persisted model and delete item attributes with nil values

[andrykonchin]

Fix saving persisted model and delete item attributes with nil values if config.store_attribute_with_nil_value = false.

The issue was introduced in #623 by switching from PutItem to UpdateItem for persisted models.

Closes #679

[github-actions]

Package	Line Rate	Health
dynamoid	90%	➖
Summary	90% (2857 / 3162)	➖

Minimum allowed line rate is 90%

[codecov]

Codecov Report

Merging #681 (9da17f3) into master (7896f96) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 9da17f3 differs from pull request most recent head eb18c4a. Consider uploading reports for the commit eb18c4a to get more accurate results

@@            Coverage Diff             @@
##           master     #681      +/-   ##
==========================================
+ Coverage   90.33%   90.35%   +0.01%     
==========================================
  Files          62       62              
  Lines        3156     3162       +6     
==========================================
+ Hits         2851     2857       +6     
  Misses        305      305              

Files Changed	Coverage Δ
...dynamoid/adapter_plugin/aws_sdk_v3/item_updater.rb	100.00% <100.00%> (ø)