Bug 1401813 - Rename Null[C]String() as Void[C]String(). r=erahm.

XPCOM's string API doesn't have the notion of a "null string". But it does have
the notion of a "void string" (or "voided string"), and that's what these
functions are returning. So the names should reflect that.

dom/html/HTMLFormElement.cpp

@@ -802,7 +802,7 @@ HTMLFormElement::SubmitSubmission(HTMLFormSubmission* aFormSubmission)
 
     rv = linkHandler->OnLinkClickSync(this, actionURI,
                                       target.get(),
-                                      NullString(),
+                                      VoidString(),
                                       postDataStream, nullptr, false,
                                       getter_AddRefs(docShell),
                                       getter_AddRefs(mSubmittingRequest));