[REFACTOR] Implement validator for registration

lib/page/registrasi/registrasi.dart

@@ -21,6 +21,7 @@ class RegistrasiState extends State<Registrasi> {
         ),
         body: SingleChildScrollView(
           child: Form(
+            key: _formKey,
             child: Center(
               child: Container(
                 margin: EdgeInsets.symmetric(horizontal: 20.0),
@@ -40,10 +41,12 @@ class RegistrasiState extends State<Registrasi> {
                     CustomTextField(
                       title: 'Nama Lengkap',
                       key: Key('Text Field Nama'),
+                      validator: (input) => input.isEmpty ? "*Wajib diisi" : null,
                     ),
                     CustomTextField(
                       title: 'Nomor Telepon',
                       key: Key('Text Field Nomor Telepon'),
+                      validator: (input) => input.isEmpty ? "*Wajib diisi" : null,
                     ),
                     CustomTextField(
                       title: 'Email (Opsional)',
@@ -52,10 +55,12 @@ class RegistrasiState extends State<Registrasi> {
                     CustomTextField(
                       title: 'Password',
                       key: Key('Text Field Password'),
+                      validator: (input) => input.isEmpty ? "*Wajib diisi" : null,
                     ),
                     CustomTextField(
                       title: 'Konfirmasi Password',
                       key: Key('Text Field Konfirmasi Password'),
+                      validator: (input) => input.isEmpty ? "*Wajib diisi" : null,
                     ),
                     Container(
                         margin: EdgeInsets.fromLTRB(0, 30, 0, 10),