[dart2] mustache templates minor improvements

Created by: alespour

Fixes two nits in dart2 mustache templates that should satisfy linter:

• add @override toString (rule annotate_overrides)
• use String's isNotEmpty instead of !isEmpty (rule prefer_is_not_emty)

@kuhnroyal

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

  Commit all changed files. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master, 5.1.x, 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

modules/openapi-generator/src/main/resources/dart2/api_exception.mustache

@@ -10,6 +10,7 @@ class ApiException implements Exception {
   Exception innerException;
   StackTrace stackTrace;
 
+  @override
   String toString() {
     if (message == null) {
       return 'ApiException';

modules/openapi-generator/src/main/resources/dart2/api_exception.mustache

@@ -10,6 +10,7 @@ class ApiException implements Exception {
   Exception innerException;
   StackTrace stackTrace;
 
+  @override
   String toString() {
     if (message == null) {
       return 'ApiException';

modules/openapi-generator/src/main/resources/dart2/api_helper.mustache

@@ -19,7 +19,7 @@ Iterable<QueryParam> _convertParametersForCollectionFormat(
   final params = <QueryParam>[];
 
   // preconditions
-  if (name != null && !name.isEmpty && value != null) {
+  if (name != null && name.isNotEmpty && value != null) {
     if (value is List) {
       // get the collection format, default: csv
       collectionFormat = (collectionFormat == null || collectionFormat.isEmpty)

samples/openapi3/client/petstore/dart2/petstore_client_lib/lib/api_exception.dart

@@ -19,6 +19,7 @@ class ApiException implements Exception {
   Exception innerException;
   StackTrace stackTrace;
 
+  @override
   String toString() {
     if (message == null) {
       return 'ApiException';

samples/openapi3/client/petstore/dart2/petstore_client_lib/lib/api_helper.dart

@@ -28,7 +28,7 @@ Iterable<QueryParam> _convertParametersForCollectionFormat(
   final params = <QueryParam>[];
 
   // preconditions
-  if (name != null && !name.isEmpty && value != null) {
+  if (name != null && name.isNotEmpty && value != null) {
     if (value is List) {
       // get the collection format, default: csv
       collectionFormat = (collectionFormat == null || collectionFormat.isEmpty)

samples/openapi3/client/petstore/dart2/petstore_client_lib_fake/lib/api_exception.dart

@@ -19,6 +19,7 @@ class ApiException implements Exception {
   Exception innerException;
   StackTrace stackTrace;
 
+  @override
   String toString() {
     if (message == null) {
       return 'ApiException';

samples/openapi3/client/petstore/dart2/petstore_client_lib_fake/lib/api_helper.dart

@@ -28,7 +28,7 @@ Iterable<QueryParam> _convertParametersForCollectionFormat(
   final params = <QueryParam>[];
 
   // preconditions
-  if (name != null && !name.isEmpty && value != null) {
+  if (name != null && name.isNotEmpty && value != null) {
     if (value is List) {
       // get the collection format, default: csv
       collectionFormat = (collectionFormat == null || collectionFormat.isEmpty)

samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/api_exception.dart

@@ -19,6 +19,7 @@ class ApiException implements Exception {
   Exception innerException;
   StackTrace stackTrace;
 
+  @override
   String toString() {
     if (message == null) {
       return 'ApiException';

samples/openapi3/client/petstore/dart2/petstore_json_serializable_client_lib_fake/lib/api_helper.dart

@@ -28,7 +28,7 @@ Iterable<QueryParam> _convertParametersForCollectionFormat(
   final params = <QueryParam>[];
 
   // preconditions
-  if (name != null && !name.isEmpty && value != null) {
+  if (name != null && name.isNotEmpty && value != null) {
     if (value is List) {
       // get the collection format, default: csv
       collectionFormat = (collectionFormat == null || collectionFormat.isEmpty)