Fix #update, #update!, .update_fields, .upsert and save empty Set and String as nil

[andrykonchin]

There is a limitation of DynamoDB - a Set cannot be empty (documentation). An attempt to save such incorrect value leads to a validation error. So in Dynamoid at saving an empty Set is replaced with nil to prevent raising exception.

Here we fix some missing cases when empty Set is not replaced with nil.

There was a yet another DynamoDB limitation - we couldn't save empty String. This is not actual anymore but Dynamoid still replaces empty Strings with nil as well. It should be changed in some future release.

Fixed methods:

• #update, #update!
• .update_fields
• .upsert

[codecov]

Codecov Report

Merging #626 (56df2a5) into master (0df1e6f) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #626      +/-   ##
==========================================
+ Coverage   90.15%   90.17%   +0.01%     
==========================================
  Files          60       60              
  Lines        3048     3054       +6     
==========================================
+ Hits         2748     2754       +6     
  Misses        300      300              

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

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[github-actions]

Package	Line Rate	Health
dynamoid	90%	➖
Summary	90% (2754 / 3054)	➖

Minimum allowed line rate is 90%