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

Fixes:#26

Details

• When accessing thevalue 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. Updatelookup(..) 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