[BUG] [dart] date-time data type: Nullable value generates uncompilable code

Inside the attached openapi.yaml you'll find an 'Instant' type defined as:
Instant:
format: date-time
type: string
example: 2022-03-10T16:15:50Z
This data type is used for nullable and not nullable properties (e.g. GraphTouchResponse nullable, GraphTransactionResponse not nullable). Generating Dart code (openapi-generator generate ... -g dart) generates:

  return GraphTransactionResponse(
    uuid: mapValueOfType<String>(json, r'uuid')!,
    expires: mapDateTime(json, r'expires', '')!,
  );

and

  return GraphTouchResponse(
    uuid: mapValueOfType<String>(json, r'uuid'),
    internalId: mapValueOfType<String>(json, r'internalId'),
    expires: DateTime.fromJson(json[r'expires']),
    touched: mapValueOfType<bool>(json, r'touched'),
  );

In the latter case expires: mapDateTime(json, r'expires', ''), would be expected as it is the case with the (String) type uuid.

Version

D:\source\rinkon\clientdart>openapi-generator.bat version
0
6.0.0

OpenAPI declaration file content or url

yaml file attached inside the zip

Generation Details

D:\source\rinkon\clientdart>openapi-generator.bat generate -i {path/to/openapi.yaml} -g dart --additional-properties=pubName=client_dart,pubLibrary=client_dart.api --minimal-update

Steps to reproduce

Output of CLI command above

Related issues/PRs

Unknown

Suggest a fix

So far I do not see any particular details in DartClientCodegen.java and AbstractDartCodegen.java
openapi.zip

Start with the attached openapi.yaml and reproduce the issue using the documented openapi-generator generate command with the dart generator. Read DartClientCodegen.java and AbstractDartCodegen.java to trace nullable date-time property handling. Done means nullable date-time fields generate the same nullable mapDateTime form as the expected expires output and the generated Dart code compiles.