[lldbsuite] Fix the mac version decorator to work on non-mac platforms

Summary: On non-mac platforms, mac_ver returns an empty string which when converted to LooseVersion has no "version" property. This causes a failure when the decorator executes. Instead, check whether the value returned from mac_ver is an empty string and avoid the LooseVersion comparison.

Reviewers: labath, davide, asmith, shafik, jingham

Reviewed By: jingham

Subscribers: jingham, lldb-commits

Differential Revision: https://reviews.llvm.org/D53208

llvm-svn: 344623