IE C++: treat NULL BSTR as empty string ("")

- This is compliant with the expected semantics of BSTR:
  http://blogs.msdn.com/b/ericlippert/archive/2003/09/12/52976.aspx
- This was exposed by a recent update to IE11 (11.0.14), which perhaps also
  introduced updates to the JScript COM bridge where empty strings in JS
  are now passed via COM as NULL BSTRs.