Fix 8.4 deprecation notices

Author: patrickkerrigan

src/Segment.php

@@ -196,10 +196,10 @@ public function getId(): string
     }
 
     /**
-     * @param string $parentId
+     * @param string|null $parentId
      * @return static
      */
-    public function setParentId(string $parentId = null)
+    public function setParentId(?string $parentId = null)
     {
         $this->parentId = $parentId;
 

src/Trace.php

@@ -46,10 +46,10 @@ public static function getInstance()
     }
 
     /**
-     * @param string $traceHeader
+     * @param string|null $traceHeader
      * @return static
      */
-    public function setTraceHeader(string $traceHeader = null)
+    public function setTraceHeader(?string $traceHeader = null)
     {
         if ($traceHeader === null) {
             return $this;