lockfile: fix no-Gemfile case

I was seeing a crash locally analyzing a repo with no Gemfile. It
appears that Bundler's LockfileParser was actually trying to read a file
somewhere even though we gave it string contents.

This changes the no-file behavior to be a harder short-circuit, so
Bundler's class is never even instantiated.

Author: wfleming

lib/cc/engine/lockfile_specs.rb

@@ -2,14 +2,11 @@
 
 class LockfileSpecs
   def initialize(lockfile)
-    if File.exist?(lockfile)
-      @contents = File.read(lockfile)
-    else
-      @contents = ""
-    end
+    @contents = File.read(lockfile) if File.exist?(lockfile)
   end
 
   def include?(name)
+    return false if @contents.nil?
     @lockfile ||= Bundler::LockfileParser.new(@contents)
     @lockfile.specs.any? { |spec| spec.name == name }
   rescue Bundler::LockfileError