Fix implicitly nullable type for $mode in render()

Author: robskr

src/Renderers/Bs3FormRenderer.php

@@ -42,7 +42,7 @@ public function __construct()
 	}
 
 
-	public function render(Form $form, string $mode = null): string
+	public function render(Form $form, ?string $mode = null): string
 	{
 		if (!isset($this->form) || $this->form !== $form) {
 			$this->controlsInit = false;

src/Renderers/Bs4FormRenderer.php

@@ -63,7 +63,7 @@ public function __construct(string $layout = FormLayout::HORIZONTAL)
 	}
 
 
-	public function render(Form $form, string $mode = null): string
+	public function render(Form $form, ?string $mode = null): string
 	{
 		if (!isset($this->form) || $this->form !== $form) {
 			$this->controlsInit = false;

src/Renderers/Bs5FormRenderer.php

@@ -65,7 +65,7 @@ public function __construct(string $layout = FormLayout::HORIZONTAL)
 	}
 
 
-	public function render(Form $form, string $mode = null): string
+	public function render(Form $form, ?string $mode = null): string
 	{
 		if (!isset($this->form) || $this->form !== $form) {
 			$this->controlsInit = false;