Xcode: Fix single-configuration generation for version 1.5

In commit 84fdc992 (stringapi: Pass configuration names as strings,
2014-02-09) a few code paths for the Xcode 1.5 single-configuration
generator were not updated to use an empty configuration name instead of
a NULL pointer when no configuration is specified in CMAKE_BUILD_TYPE.
Fix them now.
Merge topic 'xcode-15-string-apis' into next

23dc6aa1 Xcode: Fix single-configuration generation for version 1.5