fix(Multipart): properly handle falsey values like "0" (#3253)

Co-authored-by: Sean O'Brien <60306702+stobrien89@users.noreply.github.com>

Author: joshtrichards

.changes/nextrelease/multipart-falsy-values.json

@@ -0,0 +1,7 @@
+[
+  {
+    "type": "bugfix",
+    "category": "Multipart",
+    "description": "Fixes bug in `AbstractUploadManager` where valid falsy values are excluded."
+  }
+]

src/Multipart/AbstractUploadManager.php

@@ -240,7 +240,7 @@ private function determineState(): UploadState
         $id = [$required['upload_id'] => null];
         unset($required['upload_id']);
         foreach ($required as $key => $param) {
-            if (!$this->config[$key]) {
+            if (!isset($this->config[$key]) || $this->config[$key] === '') {
                 throw new IAE('You must provide a value for "' . $key . '" in '
                     . 'your config for the MultipartUploader for '
                     . $this->client->getApi()->getServiceFullName() . '.');