add basic test and fix default validator

packages/stream_chat_flutter_core/test/message_input_controller_test.dart

@@ -0,0 +1,30 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:stream_chat_flutter_core/stream_chat_flutter_core.dart';
+
+void main() {
+  testWidgets(
+    'should instantiate a new MessageInputController with default validator'
+    ' and empty message',
+    (tester) async {
+      final controller = MessageInputController();
+
+      expect(controller.isValid, false);
+      controller.text = 'test';
+      expect(controller.isValid, true);
+    },
+  );
+
+  testWidgets(
+    'should instantiate a new MessageInputController with default validator'
+    ' and specified message',
+    (tester) async {
+      final message = Message(text: 'test');
+      final controller = MessageInputController(
+        message: message,
+      );
+
+      expect(controller.message, message);
+      expect(controller.isValid, true);
+    },
+  );
+}