[BUGFIX] Consistent return types for condition-based ViewHelpers

In most cases, condition-based ViewHelpers already return an empty string
as default value, for example if the verdict is false and else is not set. However,
there are a few cases where `null` would be returned instead, which would
in the template implicitly converted to an empty string in the vast majority of
cases anyway.

This change makes sure that all closures or direct ViewHelper node evaluations
default to an empty string if `null` is returned, making the behavior consistent
to other code paths where an empty string is returned explicitly.

src/Core/ViewHelper/AbstractConditionViewHelper.php

@@ -97,7 +97,7 @@ protected function renderThenChild()
 
         // Closure might be present if ViewHelper is called from a cached template
         if ($this->thenClosure !== null) {
-            return ($this->thenClosure)();
+            return ($this->thenClosure)() ?? '';
         }
 
         // The following code can only be evaluated for uncached templates where the node structure
@@ -123,7 +123,7 @@ protected function renderThenChild()
         if ($elseViewHelperEncountered) {
             return '';
         }
-        return $this->renderChildren();
+        return $this->renderChildren() ?? '';
     }
 
     /**
@@ -142,7 +142,7 @@ protected function renderElseChild()
             // the "body" closure if condition is met
             foreach ($this->elseIfClosures as $elseIf) {
                 if ($elseIf['condition']()) {
-                    return $elseIf['body']();
+                    return $elseIf['body']() ?? '';
                 }
             }
         }
@@ -156,7 +156,7 @@ protected function renderElseChild()
 
         // Closure might be present if ViewHelper is called from a cached template
         if ($this->elseClosure !== null) {
-            return ($this->elseClosure)();
+            return ($this->elseClosure)() ?? '';
         }
 
         // The following code can only be evaluated for uncached templates where the node structure
@@ -188,7 +188,7 @@ protected function renderElseChild()
             return $this->arguments['else'];
         }
 
-        return $elseNode instanceof ViewHelperNode ? $elseNode->evaluate($this->renderingContext) : '';
+        return $elseNode instanceof ViewHelperNode ? $elseNode->evaluate($this->renderingContext) ?? '' : '';
     }
 
     /**

tests/Functional/ViewHelpers/IfThenElseViewHelperTest.php

@@ -54,7 +54,7 @@ public static function renderDataProvider(): \Generator
         yield 'else argument, verdict true' => [
             '<f:if condition="{verdict}" else="elseArgument" />',
             ['verdict' => true],
-            null,
+            '',
         ];
         yield 'else argument, verdict false' => [
             '<f:if condition="{verdict}" else="elseArgument" />',
@@ -396,8 +396,7 @@ public static function renderDataProvider(): \Generator
         yield 'inline syntax, else argument, verdict true' => [
             '{f:if(condition:\'{verdict}\', else:\'elseArgument\')}',
             ['verdict' => true],
-            // @todo: wtf?
-            null,
+            '',
         ];
         yield 'inline syntax, else argument, verdict false' => [
             '{f:if(condition:\'{verdict}\', else:\'elseArgument\')}',