update database migrations.

Author: lilianjin

database/migrations/2018_12_25_081029_create_configurations_table.php

@@ -17,7 +17,7 @@ public function up()
             $table->increments('id');
             $table->string('scope');
             $table->string('key');
-            $table->string('value');
+            $table->string('value')->nullable();
         });
     }