Validate empty pubspec plugin section (#51503)

packages/flutter_tools/lib/src/plugins.dart

@@ -177,6 +177,9 @@ class Plugin {
   }
 
   static List<String> validatePluginYaml(YamlMap yaml) {
+    if (yaml == null) {
+      return <String>['Invalid "plugin" specification.'];
+    }
 
     final bool usesOldPluginFormat = const <String>{
       'androidPackage',

packages/flutter_tools/test/general.shard/plugin_parsing_test.dart

@@ -158,6 +158,16 @@ void main() {
       expect(plugin.platforms, <String, PluginPlatform> {});
     });
 
+    test('error on empty plugin', () {
+      const String pluginYamlRaw = '';
+
+      final YamlMap pluginYaml = loadYaml(pluginYamlRaw) as YamlMap;
+      expect(
+            () => Plugin.fromYaml(_kTestPluginName, _kTestPluginPath, pluginYaml, const <String>[]),
+        throwsToolExit(message: 'Invalid "plugin" specification.'),
+      );
+    });
+
     test('error on empty platforms', () {
       const String pluginYamlRaw = 'platforms:\n';