VS: Fix use of Windows7.1SDK 64-bit toolset with VS 2010 Express

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.

Source/cmGlobalVisualStudio10Generator.cxx

@@ -339,7 +339,11 @@ void cmGlobalVisualStudio10Generator::EnableLanguage(
 
 const char* cmGlobalVisualStudio10Generator::GetPlatformToolset() const
 {
-  return this->GetPlatformToolsetString().c_str();
+  std::string const& toolset = this->GetPlatformToolsetString();
+  if (toolset.empty()) {
+    return CM_NULLPTR;
+  }
+  return toolset.c_str();
 }
 
 std::string const& cmGlobalVisualStudio10Generator::GetPlatformToolsetString()