There were two problems with this. One, I had to suppress registering build platforms in UBT if the project has no code. To achieve this I added new flag -nocodeproject and I'm passing it down from the editor to UAT.
The second one emerged after fixing previous one. The Editor was adding -build flag to the UAT if it found any of the VS 2012 or 2013. -build flag is making UAT to try to build, which fails as it is not supported on VS 2012-only system. During the discussion with platform team we came up with the solution to not check for VS 2012 in editor, as we only support it for XDK development, which requires both VSes on the system (2012 and 2013). Hence flag -build should be appended to the UAT command line if and only if there is VS2013 on the system.
[CL 2431415 by Jaroslaw Palczynski in Main branch]
