Fixed migration

database/migrations/2019_07_07_230040_create_professors_table.php

             $table->string('num_prof', 16);
             $table->string('first_name');
             $table->string('last_name');
-            $table->string('system_name');
+            $table->string('system_name')->nullable();
             $table->string('email')->nullable();
             $table->enum('type', ['plantilla', 'contrato', 'ta'])->nullable();
             $table->unsignedBigInteger('dept_id')->default(0);