[TASK] Update errorHandler property type annotation

In RenderingContext: Property $errorHandler can be
null if setErrorHandler() has not been called.
Reflect this in the type annotation.

phpstan-baseline.neon

@@ -24,12 +24,6 @@ parameters:
 			count: 1
 			path: src/Core/Parser/TemplateParser.php
 
-		-
-			message: '#^Property TYPO3Fluid\\Fluid\\Core\\Rendering\\RenderingContext\:\:\$errorHandler \(TYPO3Fluid\\Fluid\\Core\\ErrorHandler\\ErrorHandlerInterface\) in isset\(\) is not nullable\.$#'
-			identifier: isset.property
-			count: 1
-			path: src/Core/Rendering/RenderingContext.php
-
 		-
 			message: '#^Strict comparison using \!\=\= between string and null will always evaluate to true\.$#'
 			identifier: notIdentical.alwaysTrue

src/Core/Rendering/RenderingContext.php

@@ -36,7 +36,7 @@
 class RenderingContext implements RenderingContextInterface
 {
     /**
-     * @var ErrorHandlerInterface
+     * @var ErrorHandlerInterface|null
      */
     protected $errorHandler;