[Juno] assertion failure on empty input

A ProgramNode is created where the SourceRange is [buffer.end(), buffer.end()). When buffer.end() is passed to Hermes findBufferAndLine(), it works by returning an 0-length DataRef [end(), 0) as a line. Juno can't deal with this because it expects the location to be *inside* the line, so it crashes.

Also reported externally in https://github.com/facebook/hermes/pull/595#issuecomment-961859809 .

Opening this issue for public tracking of the bug.