Cast null to a type when it is a result of an empty Option (necessary…

[joezini]

… for DB2 SQL)

Casting only seemed to be necessary for Constant Columns that were options. The addTypingToLiteralColumn method (formerly addTypingToSqlColumn) seems to be necessary because to keep existing tests passing we should only be casting literals that are inside scalar functions and joins, so this function is called from those cases only.

I wanted to use OptionColumnType#isNull to check whether the column's value is null rather than checking explicitly for equality with None or null, but it always threw these type errors when I passed the column's value to that method:

[error]  found   : constantColumn.value.type (with underlying type _)
[error]  required: optionColumnType.baseColumnType.type

[codecov-io]

Codecov Report

Merging #79 into master will increase coverage by 0.13%. The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master      #79      +/-   ##
==========================================
+ Coverage   81.69%   81.82%   +0.13%     
==========================================
  Files          43       43              
  Lines        1147     1150       +3     
  Branches      105       98       -7     
==========================================
+ Hits          937      941       +4     
+ Misses        210      209       -1

Impacted Files	Coverage Δ
...rc/main/scala/sqlest/sql/DB2StatementBuilder.scala	83.6% <88.88%> (+2.57%)	:arrow_up:

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[joezini]

It turned out this change didn't work so I'm gonna close the pull request - it failed a test in VersioningSpec of AS by casting nulls that had already been cast one, and still failing to cast nulls that are the default value of an Option column that has been versioned at the column level.

Perhaps a better place for the fix is in VersionedStatementBuilder (in AS) instead? I started on this but hadn't got anything working yet, and am now being taken off the project so can't work on it any more :(

[DavidGregory084]

Okay, thanks @joezini - hopefully we can get back to this soon