fix: Handle nullable enums

[Piszmog]

When an enum is nullable, update the kotlin code to handle those cases.

Fixes: #26

Details

• When accessing the value of a nullable enum function parameter, use ?. instead of .. Using . is a compile error
• When looking up the enum value from a string, since the column is nullable, getString(..) can return null. Update lookup(..) to allow null values.
• If the enum lookup returns a null value, do not use !! on it

Appendix

• Update examples with nullable enum Piszmog/sqlc-gen-kotlin#1
  • Examples updated based on this PR