fix build error which it was occurred if config.targets is empty

This should fix following error

    NoMethodError: undefined method `empty?' for nil:NilClass
    /Users/watson/prj/RubyMotion/lib/motion/project/builder.rb:238:in `build'
    /Users/watson/prj/RubyMotion/lib/motion/project/app.rb:78:in `build'
    /Users/watson/prj/RubyMotion/lib/motion/project/template/ios.rb:68:in `block (2 levels) in <top (required)>'
[vendor] Traverse symlink dirs when finding headers.
Revert "[osx] Merge all Info.plist data in one place and ensure there's always a principal class."

This reverts commit facf63d0248fbe656e6fa54f8e93c747778f0a0e.
Revert "[osx] Allow the user to configure a NSApplication subclass and use that instead."

This reverts commit 51538c0ac769c7e49af64c9b18d0a2c60ce1712d.

Conflicts:
	test/test/spec/main_spec.rb