Fallback to about:blank to fix bustage of document.open() in consumers such as View MathML Source, b=289363 r+sr=jst, a=asa

content/html/document/src/nsHTMLDocument.cpp

@@ -1867,7 +1867,8 @@ nsHTMLDocument::OpenCommon(const nsACString& aContentType, PRBool aReplace)
 
   if (callerPrincipal) {
     callerPrincipal->GetURI(getter_AddRefs(uri));
-  } else {
+  }
+  if (!uri) {
     rv = NS_NewURI(getter_AddRefs(uri),
                    NS_LITERAL_CSTRING("about:blank"));
     NS_ENSURE_SUCCESS(rv, rv);