Fix error on empty response headers

Fixes mozilla#18957

mozilla#18682 introduced a regression that causes the following error:

```
Uncaught TypeError: Failed to construct 'Headers': Invalid name
    at PDFNetworkStreamFullRequestReader._onHeadersReceived (pdf.mjs:10214:29)
    at NetworkManager.onStateChange (pdf.mjs:10103:22)
```

The mentioned PR replaced a call to `getResponseHeader()` with `getAllResponseHeaders()` without handling cases where it may return null or an empty string. Quote from the [docs](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/getAllResponseHeaders#return_value):

> Returns:
>
>A string representing all of the response's headers (except those whose field name is Set-Cookie) separated by [CRLF](https://developer.mozilla.org/en-US/docs/Glossary/CRLF), or null if no response has been received. If a network error happened, an empty string is returned.

src/display/network.js

@@ -273,16 +273,17 @@ class PDFNetworkStreamFullRequestReader {
     const fullRequestXhrId = this._fullRequestId;
     const fullRequestXhr = this._manager.getRequestXhr(fullRequestXhrId);
 
-    const responseHeaders = new Headers(
-      fullRequestXhr
-        .getAllResponseHeaders()
-        .trim()
-        .split(/[\r\n]+/)
-        .map(x => {
-          const [key, ...val] = x.split(": ");
-          return [key, val.join(": ")];
-        })
-    );
+    const rawResponseHeaders = fullRequestXhr.getAllResponseHeaders();
+    const responseHeadersMap = rawResponseHeaders
+      ? rawResponseHeaders
+          .trim()
+          .split(/[\r\n]+/)
+          .map(x => {
+            const [key, ...val] = x.split(": ");
+            return [key, val.join(": ")];
+          })
+      : [];
+    const responseHeaders = new Headers(responseHeadersMap);
 
     const { allowRangeRequests, suggestedLength } =
       validateRangeRequestCapabilities({