XYChart2: Make min/max point size conditional on size field mapping

[leeoniya]

the point size slider is the common/shared point size when there is no size field mapped. this size can be overridden per series y field. the min/max point size inputs only apply to fields that have a size field mapped. we should hide them if no size field is mapped in any series.

similar behavior for color field. we currently don't expose the color controls at all and expect people to rely on y series overrides for static colors. if a color field is mapped we rely on overrides for the mapped color field (scheme, value mappings, thresholds). this might be tackled as part of #85025.