Fix option value can accept null

app/Services/Option.php

@@ -45,12 +45,16 @@ class Option
      * Set new value for given option key.
      *
      * @param string $key   The option key.
-     * @param string $value The option value to be saved.
+     * @param mixed $value The option value to be saved.
      *
      * @return string The option value.
      */
-    public function set($key, string $value)
+    public function set($key, $value)
     {
+        if (is_null($value) || !is_string($value)) {
+            $value = '';
+        }
+
         $option = $this->option->where('key', $key)->first();
 
         if ($option) {

tests/Unit/Services/SiteOptionTest.php

@@ -22,6 +22,17 @@ class SiteOptionTest extends TestCase
     }
 
     /** @test */
+    public function option_value_null_must_be_converted_to_empty_string()
+    {
+        Option::set('testing_key', null);
+
+        $this->seeInDatabase('site_options', [
+            'key'   => 'testing_key',
+            'value' => '',
+        ]);
+    }
+
+    /** @test */
     public function option_can_be_get()
     {
         \DB::table('site_options')->insert([