Change CommandReturnObject::AppendMessage to print an empty line if
AppendMessage("") is called.  This idiom is used in a handful of places
right now (e.g. to put space between different threads in 'bt all') but
the empty newline is being omitted instead of emitted.
<rdar://problem/13753830> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@180841 91177308-0d34-0410-b5e6-96231b3b80d8