fix: setting a field to null does not delete it via GraphQL API (#7649)

BREAKING CHANGE: To delete a field via the GraphQL API, the field value has to be set to `null`. Previously, setting a field value to `null` would save a null value in the database, which was not according to the [GraphQL specs](https://spec.graphql.org/June2018/#sec-Null-Value). To delete a file field use `file: null`, the previous way of using `file: { file: null }` has become obsolete.

CHANGELOG.md

@@ -101,7 +101,7 @@ ___
 # [Unreleased (master branch)](https://github.com/parse-community/parse-server/compare/5.0.0-alpha.1...master)
 
 ## Breaking Changes
-  - (none)
+  - feat: `null` value on field during graphql mutation now unset the value from the database, file unset changed
 ## Features
   - (none)
 ## Bug Fixes