🐛 Fix the default build file definition

Author: AlexV525

packages/command/bin/flutter_gen_command.dart

@@ -18,7 +18,6 @@ void main(List<String> args) {
     'build',
     abbr: 'b',
     help: 'Set the path of build.yaml.',
-    defaultsTo: 'build.yaml',
   );
 
   parser.addFlag(
@@ -57,11 +56,11 @@ void main(List<String> args) {
   }
   final pubspecFile = File(pubspecPath).absolute;
 
-  final buildPath = safeCast<String>(results['build']);
-  if (buildPath == null || buildPath.trim().isEmpty) {
+  final buildPath = safeCast<String>(results['build'])?.trim();
+  if (buildPath?.isEmpty ?? false) {
     throw ArgumentError('Invalid value $buildPath', 'build');
   }
-  final buildFile = File(buildPath).absolute;
+  final buildFile = buildPath == null ? null : File(buildPath).absolute;
 
   FlutterGenerator(pubspecFile, buildFile: buildFile).build();
 }

packages/core/lib/settings/config.dart

@@ -32,6 +32,12 @@ Config loadPubspecConfig(File pubspecFile, {File? buildFile}) {
     '[FlutterGen] Reading options from $pubspecLocaleHint',
   );
 
+  // Fallback to the build.yaml when no build file has been specified and
+  // the default one exists.
+  if (buildFile == null && File('build.yaml').existsSync()) {
+    buildFile = File('build.yaml');
+  }
+
   if (buildFile != null) {
     if (buildFile.existsSync()) {
       final buildContent = buildFile.readAsStringSync();