[BUG]: sankey: node.x / node.y of 0 is ignored #8150

Description

Node coordinates of 0 for node.x or node.yare ignored and the node gets positioned automatically.

Screenshots

With 0 coords:

With close to 0 coords:

Steps to reproduce

• Open Plotly devtools and paste the following into browser devtools:

Plotly.newPlot('g', [{
  type: 'sankey',
  arrangement: 'fixed',
  node: {label: ['a', 'b', 'c'], x: [0, 0.5, 0.8], y: [0, 0, 0.7]},
  link: {source: [0, 1], target: [1, 2], value: [1, 1]}
}], {margin: {l: 0, r: 0, t: 0, b: 0}});

• Note that the center of a is not in the top left corner of the plot

Notes

• This issue stems from the fact that the "Force node position" loop in src/traces/sankey/render.js uses a truthy test (if(trace.node.x[p] && trace.node.y[p])), and 0 is falsy
• The fix is to change the test for missing values to != null
• The mock sankey_x_y_with_arrows.json sets y: [0, 0, 0], so the fix changes its baseline
• Discovered while working on Respect explicit node x/y positions in fixed-arrangement Sankey #7826