Fix null input handling

[CarsonF]

Handle GraphQL nulls from "optional" fields as undefined.

GraphQL's / Nest's decorators have nullable option, which means the value can be null or omitted - which naturally translates to null or undefined in JS.

We actually care about this distinction, though, especially on update mutations.
undefined means no change.
null means clear the value.

We carelessly allowed both meaningly that nulls could mistakenly be stored in lots of spots that are unexpected.
Granted, this only would happen in Neo4j, as Gel's schema checks would prevent this.

Going forward we have a new optional option as a sibling to nullable.
optional lets the field be omitted or undefined. GraphQL can still give us nulls, but be will we convert them to undefined.
nullable continues to let null & undefined through.

---

I also added null types where we actually expect/allow nulls.

I also created ListField to help dedupe items, and convert empty lists to undefined (for filters), and to disallow empty arrays where expected.

[github-actions]

🗞 GraphQL Summary

View schema changes

@@ -2601,9 +2601,9 @@
   presetInventory: Boolean
   registryOfDialectsCode: String
   registryOfLanguageVarietiesCode: String
 
-  """Only languages with these sensitivities"""
+  """Only these sensitivities"""
   sensitivity: [Sensitivity!]
 }
 
 input LanguageListInput {
@@ -4861,9 +4861,9 @@
   presetInventory: Boolean
   primaryLocation: LocationFilters
   primaryPartnership: PartnershipFilters
 
-  """Only projects with these sensitivities"""
+  """Only these sensitivities"""
   sensitivity: [Sensitivity!]
 
   """Only projects matching these statuses"""
   status: [ProjectStatus!]