polish: handling for empty next_cursor

- and add return type to StartCursor '__toString'

Author: johguentner

src/Entities/Collections/EntityCollection.php

@@ -170,8 +170,10 @@ public function hasMoreEntries(): bool
     /**
      * @return StartCursor
      */
-    public function nextCursor(): StartCursor
+    public function nextCursor(): ?StartCursor
     {
-        return new StartCursor($this->getRawNextCursor());
+        $rawNextCursor = $this->getRawNextCursor();
+        if($rawNextCursor === null) return null;
+        return new StartCursor($rawNextCursor);
     }
 }

src/Query/StartCursor.php

@@ -22,7 +22,7 @@ public function __construct(string $cursor)
         $this->cursor = $cursor;
     }
 
-    public function __toString()
+    public function __toString(): string
     {
         return $this->cursor;
     }