fix ghost fill-tonext when either trace is emptied out

[alexcjohnson]

Fixes #2271 - we were short-circuiting a little too much behavior when no line is generated by a trace.

@dfcreative another quick one to look at, if you would!

[dy]

Turns out this test case does not work with scattergl, although it should, I should have a look.
Also is that correct that the legend shows an empty trace?

In all looks good!

[alexcjohnson]

is that correct that the legend shows an empty trace?

Yes, trace 1 has mode: 'lines', line: {width: 0}, rendering it invisible (could have just done mode: 'none' like trace 2 but without the fill, but this works too)

[dy]

💃 :)