[lldb] Fix TestSettings.test_pass_host_env_vars on windows

Summary:
A defensive check in ProcessLauncherWindows meant that we would never
attempt to launch a process with a completely empty environment -- the
host environment would be used instead. Instead, I make the function add
an extra null wchar_t at the end of an empty environment. The
documentation on this is a bit fuzzy, but it seems to be what is needed
to make windows accept these kinds of environments.

Reviewers: amccarth, friss

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D76835