fix: Strip out empty srcSet entries passed to renderFigure()

[joostdecock]

Some of my images where not showing up, so I did some digging and
noticed that an array like this was passed to renderFigure():

[
  { aspectRatio: 0, srcSet: [], width: 1000 },
  { aspectRatio: 0, srcSet: [], width: 2000 },
  {
    aspectRatio: 1.4970059880239521,
    srcSet: [ [Object], [Object] ],
    width: 250
  },
  { aspectRatio: 1.5015015015015014, srcSet: [ [Object] ], width: 500 }
]

In renderFigure() we use srcSets.shift().srcSet which in a case like
this means the img tag will be skipped.

This filters the sources list to only those that have an actual srcSet
value. The array passed to renderFigure() becomes:

[
  {
    aspectRatio: 1.4970059880239521,
    srcSet: [ [Object], [Object] ],
    width: 250
  },
  { aspectRatio: 1.5015015015015014, srcSet: [ [Object] ], width: 500 }
]