Restore is_string() as a Smarty modifier

- Address https://github.com/friendica/friendica/issues/13157#issuecomment-1567336694

src/Render/FriendicaSmarty.php

@@ -53,6 +53,10 @@ class FriendicaSmarty extends Smarty
 		$this->setConfigDir($work_dir . '/');
 		$this->setCacheDir($work_dir . '/');
 
+		$this->registerPlugin('modifier', 'is_string', function ($value) {
+			return is_string($value);
+		});
+
 		/*
 		 * Enable sub-directory splitting for reducing directory descriptor
 		 * size. The default behavior is to put all compiled/cached files into

view/templates/settings/addons/panel.tpl

@@ -8,7 +8,7 @@
 	{{$html nofilter}}
 {{if $submit}}
 	<div class="settings-submit-wrapper panel-footer">
-    {{if $submit}}
+    {{if $submit|is_string}}
 		<button type="submit" name="{{$addon}}-submit" class="btn btn-primary settings-submit" value="{{$submit}}">{{$submit}}</button>
     {{else}}
         {{$count = 1}}

view/theme/frio/templates/settings/addons/panel.tpl

@@ -11,7 +11,7 @@
 		</div>
 		<div class="panel-footer">
 {{if $submit}}
-    {{if $submit}}
+    {{if $submit|is_string}}
 			<button type="submit" name="{{$addon}}-submit" class="btn btn-primary settings-submit" value="{{$submit}}">{{$submit}}</button>
     {{else}}
         {{$count = 1}}