BUGFIX: 3585 Fusion Runtime @apply with integer indexed array

starting with PHP 7.4 it's not cool to do offset access on integers (which returns null always)

I did a little digging, and we have this skip keys logic 3 times in the runtime (probably not extracted for performance reasons)

2 times this logic needs to be adjusted to only operate on strings. Those two snippets where introduced with neos#2738

And 1 time this logic already has an is_string check. https://github.com/neos/neos-development-collection/blob/046e5d02750af0ebf33cc52663799ce09683ba37/Neos.Fusion/Classes/Core/Runtime.php#L667
This was adjusted when 7.4 compatibility was introduced via neos#2804 but the other two references where not adjusted

Neos.Fusion/Classes/Core/Runtime.php

@@ -761,7 +761,7 @@ protected function evaluateApplyValues($configurationWithEventualProperties, $fu
             $positionalArraySorter = new PositionalArraySorter($propertiesConfiguration, '__meta.position');
             foreach ($positionalArraySorter->getSortedKeys() as $key) {
                 // skip keys which start with __, as they are purely internal.
-                if ($key[0] === '_' && $key[1] === '_' && in_array($key, Parser::$reservedParseTreeKeys, true)) {
+                if (is_string($key) && $key[0] === '_' && $key[1] === '_' && in_array($key, Parser::$reservedParseTreeKeys, true)) {
                     continue;
                 }
 
@@ -777,7 +777,7 @@ protected function evaluateApplyValues($configurationWithEventualProperties, $fu
                     if (is_array($singleApplyValues)) {
                         foreach ($singleApplyValues as $key => $value) {
                             // skip keys which start with __, as they are purely internal.
-                            if ($key[0] === '_' && $key[1] === '_' && in_array($key, Parser::$reservedParseTreeKeys, true)) {
+                            if (is_string($key) && $key[0] === '_' && $key[1] === '_' && in_array($key, Parser::$reservedParseTreeKeys, true)) {
                                 continue;
                             }