TBB can't handle the no-edge graph

[dglazkov]

Given digraph foo { dummy }, the venerable TBB vomits out:

/Users/dglazkov/Documents/code/tree-builder-builder/gulpfile.js:37
    stageLoader.processStages(stageList, cb, function(e) { throw e; });
                                                                 ^
TypeError: Cannot read property 'v' of undefined
    at Object.impl (/Users/dglazkov/Documents/code/tree-builder-builder/core/experiment.js:47:44)
    at /Users/dglazkov/Documents/code/tree-builder-builder/core/stream.js:225:13
    at /Users/dglazkov/Documents/code/tree-builder-builder/core/stream.js:195:13
    at CoreStreamBase.fn (/Users/dglazkov/Documents/code/tree-builder-builder/core/stream.js:199:7)
    at CoreStreamBase.CoreStream.impl (/Users/dglazkov/Documents/code/tree-builder-builder/core/stream.js:175:8)
    at CoreStreamBase.stage.impl (/Users/dglazkov/Documents/code/tree-builder-builder/core/stage-loader.js:45:19)
    at /Users/dglazkov/Documents/code/tree-builder-builder/core/stage-loader.js:82:19
    at CoreStreamBase.<anonymous> (/Users/dglazkov/Documents/code/tree-builder-builder/core/stream.js:180:5)
    at /Users/dglazkov/Documents/code/tree-builder-builder/core/stream.js:207:7
    at /Users/dglazkov/Documents/code/tree-builder-builder/core/stages.js:53:5