Return null in caching stream size if remote is null (guzzle#438)

* Return null in caching stream size if remote is null

* Added test coverage

* Fixed typo

* Fixes

src/CachingStream.php

@@ -36,7 +36,13 @@ public function __construct(
 
     public function getSize(): ?int
     {
-        return max($this->stream->getSize(), $this->remoteStream->getSize());
+        $remoteSize = $this->remoteStream->getSize();
+
+        if (null === $remoteSize) {
+            return null;
+        }
+
+        return max($this->stream->getSize(), $remoteSize);
     }
 
     public function rewind(): void