Add interface MutableHtml compatibility

Author: sukhwinder33445

src/Layout/Controls.php

@@ -43,7 +43,7 @@ public function setTabs(Tabs $tabs)
         return $this;
     }
 
-    public function isEmpty()
+    public function isEmpty(): bool
     {
         if (! parent::isEmpty()) {
             return false;

src/Widget/Ball.php

@@ -91,7 +91,7 @@ protected function assembleCssClasses(): array
      *
      * @return $this
      */
-    public function addHtml(ValidHtml ...$content): Ball
+    public function addHtml(ValidHtml ...$content): static
     {
         if (count($content) === 1 && $content[0] instanceof Text) {
             $content[0] = new HtmlElement('span', null, $content[0]);