make px compare test output a diff blank file when no image is generated:
make px compare test output a diff blank file when no image is generated:

  - so that `ls test-images-diffs` reveals
    all incorrect (not-generated + with a diff) images.

plotly/plotly.js,asolagmbh/plotly.js,plotly/plotly.js,asolagmbh/plotly.js,plotly/plotly.js,iongroup/plotly.js,iongroup/plotly.js,iongroup/plotly.js,aburato/plotly.js,plotly/plotly.js,aburato/plotly.js,asolagmbh/plotly.js,etpinard/plotly.js,etpinard/plotly.js,etpinard/plotly.js