Update LetterController to handle nullable fields and set boolean flags for acceptance and completion letters

database/migrations/2025_03_08_142237_create_acceptance_letters_table.php

@@ -19,7 +19,7 @@ return new class extends Migration
             $table->string('program_studi_mahasiswa');
             $table->string('tahun_ajaran');
             $table->date('start_date');
-            $table->date('completion_date');
+            $table->date('completion_date')->nullable();
             $table->string('lampiran')->nullable();
             $table->boolean('is_acceptance')->default(false);
             $table->string('nomor_surat_acceptance')->nullable();