[BUG][DART] Fix nested-map ! suffix to apply to whole nullable expression

Cubic review on OpenAPITools#23671 caught that the prior shape

    json[X] == null
        ? null
        : (json[X] as Map).map(...).cast<...>()!

applied the trailing ! only to the else branch, so a required
non-nullable Map<String, Map<...>> field could still be assigned null
from the ? branch.

Restructure to a single nullable expression that matches the existing
mapCastOfType pattern in the same file:

    (json[X] as Map?)?.map((k, v) => MapEntry(k as String, (v as Map).cast<String, T>()))
        {{#required}}{{^isNullable}}!{{/isNullable}}{{/required}}
        {{^required}}{{#defaultValue}} ?? <default>{{/defaultValue}}{{/required}}

Now ! correctly applies to the whole nullable expression, ?? feeds the
default consistently with the other map branches, and the optional case
collapses to a single nullable expression instead of a ternary.

Author: winrid

modules/openapi-generator/src/main/resources/dart2/serialization/native/native_class.mustache

@@ -228,9 +228,7 @@ class {{{classname}}} {
         {{{name}}}: {{items.complexType}}.mapFromJson(json[r'{{{baseName}}}']),
                   {{/items.complexType}}
                   {{^items.complexType}}
-        {{{name}}}: json[r'{{{baseName}}}'] == null
-            ? {{#defaultValue}}{{{.}}}{{/defaultValue}}{{^defaultValue}}null{{/defaultValue}}
-            : (json[r'{{{baseName}}}'] as Map).map((k, v) => MapEntry(k as String, (v as Map).cast<String, {{items.items.dataType}}>())){{#required}}{{^isNullable}}!{{/isNullable}}{{/required}},
+        {{{name}}}: (json[r'{{{baseName}}}'] as Map?)?.map((k, v) => MapEntry(k as String, (v as Map).cast<String, {{items.items.dataType}}>())){{#required}}{{^isNullable}}!{{/isNullable}}{{/required}}{{^required}}{{#defaultValue}} ?? {{{.}}}{{/defaultValue}}{{/required}},
                   {{/items.complexType}}
                 {{/items.isMap}}
                 {{^items.isMap}}

samples/openapi3/client/petstore/dart2/petstore_client_lib_fake/.openapi-generator/FILES

@@ -147,7 +147,3 @@ lib/model/test_enum.dart
 lib/model/test_inline_freeform_additional_properties_request.dart
 lib/model/user.dart
 pubspec.yaml
-test/pet_empty_metadata_test.dart
-test/pet_reaction_response_test.dart
-test/pet_reaction_status_test.dart
-test/pet_reactions_response_test.dart

samples/openapi3/client/petstore/dart2/petstore_client_lib_fake/lib/model/additional_properties_class.dart

@@ -66,9 +66,7 @@ class AdditionalPropertiesClass {
 
       return AdditionalPropertiesClass(
         mapProperty: mapCastOfType<String, String>(json, r'map_property') ?? const {},
-        mapOfMapProperty: json[r'map_of_map_property'] == null
-            ? const {}
-            : (json[r'map_of_map_property'] as Map).map((k, v) => MapEntry(k as String, (v as Map).cast<String, String>())),
+        mapOfMapProperty: (json[r'map_of_map_property'] as Map?)?.map((k, v) => MapEntry(k as String, (v as Map).cast<String, String>())) ?? const {},
         mapOfArrayInteger: json[r'map_of_array_integer'] == null
           ? const {}
             : (json[r'map_of_array_integer'] as Map<String, dynamic>).map((k, v) => MapEntry(k, v == null ? const <int>[] : (v as List).cast<int>())),

samples/openapi3/client/petstore/dart2/petstore_client_lib_fake/lib/model/map_test.dart

@@ -71,9 +71,7 @@ class MapTest {
       }());
 
       return MapTest(
-        mapMapOfString: json[r'map_map_of_string'] == null
-            ? const {}
-            : (json[r'map_map_of_string'] as Map).map((k, v) => MapEntry(k as String, (v as Map).cast<String, String>())),
+        mapMapOfString: (json[r'map_map_of_string'] as Map?)?.map((k, v) => MapEntry(k as String, (v as Map).cast<String, String>())) ?? const {},
         mapOfEnumString: mapCastOfType<String, String>(json, r'map_of_enum_string') ?? const {},
         directMap: mapCastOfType<String, bool>(json, r'direct_map') ?? const {},
         indirectMap: mapCastOfType<String, bool>(json, r'indirect_map') ?? const {},

samples/openapi3/client/petstore/dart2/petstore_client_lib_fake/lib/model/pet_reactions_response.dart

@@ -59,12 +59,8 @@ class PetReactionsResponse {
       }());
 
       return PetReactionsResponse(
-        myReacts: json[r'myReacts'] == null
-            ? const {}
-            : (json[r'myReacts'] as Map).map((k, v) => MapEntry(k as String, (v as Map).cast<String, bool>())),
-        reactionCounts: json[r'reactionCounts'] == null
-            ? const {}
-            : (json[r'reactionCounts'] as Map).map((k, v) => MapEntry(k as String, (v as Map).cast<String, int>())),
+        myReacts: (json[r'myReacts'] as Map?)?.map((k, v) => MapEntry(k as String, (v as Map).cast<String, bool>())) ?? const {},
+        reactionCounts: (json[r'reactionCounts'] as Map?)?.map((k, v) => MapEntry(k as String, (v as Map).cast<String, int>())) ?? const {},
       );
     }
     return null;