feat: Add $ifNotExists tot $set operator

[onhate]

Allow to set a value only if it does not exists on the record, a good use case is on upsert and setting a createdAt property

{ $set:  { createdAt: { $ifNotExists: date() } } }

https://docs.amazonaws.cn/en_us/amazondynamodb/latest/developerguide/Expressions.UpdateExpressions.html#Expressions.UpdateExpressions.SET.PreventingAttributeOverwrites

extra: allow undefined/null where clause (to be ignored)

[onhate]

@SamVerschueren what do you think about this one?

[codecov-io]

Codecov Report

Merging #85 (3506247) into master (4080422) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
+ Coverage   96.89%   96.94%   +0.04%     
==========================================
  Files          34       34              
  Lines         645      654       +9     
  Branches       48       49       +1     
==========================================
+ Hits          625      634       +9     
  Misses         12       12              
  Partials        8        8              

Impacted Files	Coverage Δ
src/lib/methods/base-query.ts	100.00% <100.00%> (ø)
src/lib/methods/delete-item.ts	100.00% <100.00%> (ø)
src/lib/methods/update-item.ts	100.00% <100.00%> (ø)
src/lib/utils/update.ts	100.00% <100.00%> (ø)

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[onhate]

@SamVerschueren ?

[onhate]

[onhate]

@SamVerschueren ?

[SamVerschueren]

Awesome job @onhate!