TASK: refactor API for findNodeAggregates

I forgot to change this after discussing in neos/neos-development-collection#4093
so this is the cleanup commit.

Author: skurfuerst

Classes/Projection/ContentGraph/ContentGraphInterface.php

@@ -46,8 +46,9 @@ public function getSubgraph(
     ): ContentSubgraphInterface;
 
     /**
-     * @deprecated please use {@see findRootNodeAggregates} instead
-     * @internal
+     * @api
+     * Throws exception if no root aggregate found, because a Content Repository needs at least
+     * one root node to function.
      */
     public function findRootNodeAggregateByType(
         ContentStreamId $contentStreamId,

Classes/Projection/ContentGraph/NodeAggregates.php

@@ -74,17 +74,14 @@ public function count(): int
     }
 
     /**
-     * Throws exception if no root aggregate found, because a Content Repository needs at least
-     * one root node to function.
-     *
-     * @return NodeAggregate
+     * @return NodeAggregate|null
      */
-    public function first(): NodeAggregate
+    public function first(): ?NodeAggregate
     {
         if (count($this->nodeAggregates) > 0) {
             $array = $this->nodeAggregates;
             return reset($array);
         }
-        throw new \RuntimeException('Root Node Aggregate not found');
+        return null;
     }
 }