Fix optional and null vis config values

[thinkh]

Currently, we are using different ways of null and undefined aka optional property with ?. Here is an example of the scatterplot config:

visyn_core/src/vis/scatter/interfaces.ts

Lines 3 to 21 in f67d209

	export interface IScatterConfig extends BaseVisConfig {
	type: ESupportedPlotlyVis.SCATTER;
	// Numerical columns selected for x and y axis. If 2 are selected, a normal scatter plot is shown. If more than 2 are selected, a SPLOM is shown.
	numColumnsSelected: ColumnInfo[];
	// This attribute splits the data into multiple scatter plots based on the selected column.
	facets: ColumnInfo | null;
	// This attribute can be used to directly control which subplots should be shown.
	subplots: { xColumn: ColumnInfo; yColumn: ColumnInfo; title: string }[] | undefined;
	color: ColumnInfo | null;
	numColorScaleType: ENumericalColorScaleType;
	shape: ColumnInfo | null;
	dragMode: EScatterSelectSettings;
	alphaSliderVal: number;
	showLabels: ELabelingOptions;
	showLabelLimit?: number;
	regressionLineOptions?: IRegressionLineOptions;
	showLegend?: boolean;
	labelColumns?: ColumnInfo[];
	}

We should avoid null if the property is optional and use the ? instead.

export interface IScatterConfig extends BaseVisConfig {
  type: ESupportedPlotlyVis.SCATTER;
  // Numerical columns selected for x and y axis. If 2 are selected, a normal scatter plot is shown. If more than 2 are selected, a SPLOM is shown.
  numColumnsSelected: ColumnInfo[];
  // This attribute splits the data into multiple scatter plots based on the selected column.
  facets?: ColumnInfo;
  // This attribute can be used to directly control which subplots should be shown.
  subplots?: { xColumn: ColumnInfo; yColumn: ColumnInfo; title: string }[];
  color?: ColumnInfo;
  numColorScaleType: ENumericalColorScaleType;
  shape?: ColumnInfo;
  dragMode: EScatterSelectSettings;
  alphaSliderVal: number;
  showLabels: ELabelingOptions;
  showLabelLimit?: number;
  regressionLineOptions?: IRegressionLineOptions;
  showLegend?: boolean;
  labelColumns?: ColumnInfo[];
}

By making the facets, color, and shape optional, we must also change the default config by removing the properties with null values:

export const defaultConfig: IScatterConfig = {
  type: ESupportedPlotlyVis.SCATTER,
  numColumnsSelected: [],
  numColorScaleType: ENumericalColorScaleType.SEQUENTIAL,
  dragMode: EScatterSelectSettings.RECTANGLE,
  alphaSliderVal: 0.5,
  showLabels: ELabelingOptions.NEVER,
  showLabelLimit: 50,
  regressionLineOptions: {
    type: ERegressionLineType.NONE,
    fitOptions: { order: 2, precision: 3 },
    lineStyle: defaultRegressionLineStyle,
    showStats: true,
  },
};

Changing this will change the application behavior, hence, it must be tested carefully.

This also applies to other configuration from visualization types.