msxml3: Avoid SysStringLen() testing for an empty string in ::createNode()

[Greg Geldorp]

> From: Reece Dunn <msclrhd at googlemail.com>
>
> msxml3 is not present on NT4, so CLSID_XMLHTTPRequest is not
> registered. This does not fail on the test page, as the msxml3 tests
> are skipped when msxml3.dll is not present.
>
> It would probably be better to issue a win_skip in this instance, but
> it is not relevant to your change.

TestBot uses a different algorithm to check if a test should be skipped.
Winetest.exe checks the name of the test and obtains the name of the DLL
being tested from that (msxml3_test.exe => msxml3.dll). If that DLL is
not present, winetest.exe skips the test. TestBot checks the import
table of the test executable, and if any of the statically imported DLLs
are missing it skips the test. In this case, msxml3.dll is not statically
linked, so it will not be skipped by TestBot on NT4. TestBot should
probably check the test name too.

Ge.