Corrected fallback responseHeaders value to be null. (#4110)

- The return value should match that of the function XMLHttpRequest.getAllResponseHeaders() which returns a string, or null if no response headers have been received, or empty string on a network error.

src/streaming/net/HTTPLoader.js

@@ -121,7 +121,7 @@ function HTTPLoader(cfg) {
                 const responseUrl = httpRequest.response ? httpRequest.response.responseURL : null;
                 const responseStatus = httpRequest.response ? httpRequest.response.status : null;
                 const responseHeaders = httpRequest.response && httpRequest.response.getAllResponseHeaders ? httpRequest.response.getAllResponseHeaders() :
-                    httpRequest.response ? httpRequest.response.responseHeaders : [];
+                    httpRequest.response ? httpRequest.response.responseHeaders : null;
 
                 dashMetrics.addHttpRequest(request, responseUrl, responseStatus, responseHeaders, success ? traces : null);