fix: don't iterate over undefined opts.data. (#711)

Co-authored-by: sofisl <55454395+sofisl@users.noreply.github.com>

Author: nleclerc

core/packages/gaxios/src/gaxios.ts

@@ -196,7 +196,7 @@ export class Gaxios implements FetchCompliance {
         if (opts.responseType === 'stream') {
           const response = [];
 
-          for await (const chunk of opts.data as Readable) {
+          for await (const chunk of (opts.data ?? []) as Readable) {
             response.push(chunk);
           }