fix numericopt setting

In 55a4f13 a bug was introduced that

* forced resaving the config even if it hadn't changed
* prevented setting the value to an empty string once set

lib/plugins/config/_test/Setting/SettingNumericoptTest.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace dokuwiki\plugin\config\test\Setting;
+
+/**
+ * @group plugin_config
+ * @group admin_plugins
+ * @group plugins
+ * @group bundled_plugins
+ */
+class SettingNumericoptTest extends SettingNumericTest {
+
+    /** @inheritdoc */
+    public function dataOut() {
+        return array_merge(
+            parent::dataOut(),
+            [
+                ['', "\$conf['test'] = '';\n"],
+            ]
+        );
+    }
+
+}

lib/plugins/config/core/Setting/SettingNumericopt.php

@@ -15,6 +15,8 @@ class SettingNumericopt extends SettingNumeric {
      */
     public function update($input) {
         if($input === '') {
+            if($input == $this->local) return false;
+            $this->local = $input;
             return true;
         }