Add remaining missing return types to safe methods

Author: wouterj

Process.php

@@ -912,7 +912,7 @@ public function stop(float $timeout = 10, int $signal = null): ?int
      *
      * @internal
      */
-    public function addOutput(string $line)
+    public function addOutput(string $line): void
     {
         $this->lastOutputTime = microtime(true);
 
@@ -926,7 +926,7 @@ public function addOutput(string $line)
      *
      * @internal
      */
-    public function addErrorOutput(string $line)
+    public function addErrorOutput(string $line): void
     {
         $this->lastOutputTime = microtime(true);