[Maps] Icon palettes are not working

[thomasneirynck]

Icon palettes are not being displayed on the map.

Steps

1. add sample log data
2. select by-value icon styling. select machine.os.keyword field
3. use first palette
   -> icons are not showing

They show correctly in the legend.

Custom icon palettes do work correctly.

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[nreese]

Regression caused by #60570

That PR changed DynamicIconProperty._getPaletteStops to return for custom icon ramps { fallbackSymbolId, stops } instead of { fallback, stops }. The PR failed to make the same change for provided icon palettes so they still returned { fallback, stops }.

Then _getMbIconImageExpression just returns early if fallbackSymbolId is not provide.

_getMbIconImageExpression(iconPixelSize) {
    const { stops, fallbackSymbolId } = this._getPaletteStops();

    if (stops.length < 1 || !fallbackSymbolId) {
      //occurs when no data
      return null;
    }

Typing would have caught this. For a fix, I am going to type script the related files.