isNull and isNotNull added

[alfonsorr]

Added isNull and isNotNull to column.
Part of #164

[imarios]

Hi @alfonsorr and thank you for the PR!

I am kind of skeptical with this API. Assuming that your schema is typed correctly, then anything that may be null should be Optional. So if column X of type Int can be null then it should be Option[Int]. In that case we already have isNone and isNotNone APIs that essentially do exactly this, test for nullability for columns that may be null (that is, columns that have their schema explicitly state that they are optional).

If you look at all APIs in Frameless that may return null, they make that column Optional. So they kind of follow the Sala approach to not using nulls but use Options instead.

I think this API can help only in case you incorrectly specified you schema and you missed a column that is now null where it shouldn't be ...

@OlivierBlanvillain what are your thoughts?

[codecov-io]

Codecov Report

Merging #274 into master will increase coverage by 0.22%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #274      +/-   ##
==========================================
+ Coverage   94.72%   94.94%   +0.22%     
==========================================
  Files          52       52              
  Lines         948      950       +2     
  Branches       10       11       +1     
==========================================
+ Hits          898      902       +4     
+ Misses         50       48       -2

Impacted Files	Coverage Δ
dataset/src/main/scala/frameless/TypedColumn.scala	100% <100%> (ø)	⬆️
...c/main/scala/frameless/TypedDatasetForwarded.scala	74.28% <0%> (+2.85%)	⬆️
...src/main/scala/frameless/CatalystNotNullable.scala	66.66% <0%> (+33.33%)	⬆️

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[alfonsorr]

I see your point @imarios , and it's true that is repeated. And for the other posibility, I don't see useful to use it in an incorrect schema.

I will try to contribute with other element's, thanks

[imarios]

@alfonsorr I hope this doesn't stop you for contributing! I am sure what you learned doing it (and it was done quite correctly) can be applied to your next contribution.