Refactoring in commit 584ab528 (VS: Add internal API to get platform
toolset as string, 2016-10-18) accidentally changed the semantics of the
original `cmGlobalVisualStudio10Generator::GetPlatformToolset`
implementation to return a pointer to an empty string instead of
`nullptr` when no toolset is configured.  This broke cases that tested
for `nullptr` to detect the lack of any specific toolset, such as the
call in `Find64BitTools`.  Restore the `nullptr` return in this case.