Fix nil error when Result has lines=nil for some reason

I'm pretty sure this has something to do with the possibly dubious choice of nullifying several ivars in RunEmitter#complete, but hard to tell without a failing test case.

References #1

lib/nstance/result.rb

@@ -12,7 +12,9 @@ def initialize(lines = [], status = nil, error: nil)
     end
 
     def inspect
-      log = "---\n" + lines.map { |(stream, msg)| "#{stream} | #{msg}" }.join + "---"
+      log = "---\n"
+      log += lines.map { |(stream, msg)| "#{stream} | #{msg}" }.join if lines
+      log += "---"
       if error
         log + " error: #{error.inspect}"
       else