[DSL] Add support for a nullable annotation in SQL

[c-harding]

Still need to add support in Go, which will involve treating a lot of stuff as pointers, and checking that the postgres bindings support that (how else can you set stuff to null/non-null dynamically?).

This reverts commit ec842ef on PR #332.

[codecov]

Codecov Report

Merging #333 into develop will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #333   +/-   ##
========================================
  Coverage    99.01%   99.02%           
========================================
  Files          135      135           
  Lines         2857     2860    +3     
  Branches        43       46    +3     
========================================
+ Hits          2829     2832    +3     
  Misses          28       28           

Impacted Files	Coverage Δ
src/main/scala/temple/DSL/semantics/Analyzer.scala	100.00% <100.00%> (ø)
...rc/main/scala/temple/DSL/semantics/Validator.scala	100.00% <100.00%> (ø)
...rc/main/scala/temple/builder/DatabaseBuilder.scala	100.00% <100.00%> (ø)

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