Icon Picker: Fix empty selection allowed on mandatory fields and add validation.

[engijlr]

Summary

• Implements Mandatory behavior for the Icon Picker property editor:
• Hides the “empty” option in the modal when the field is mandatory.
• Adds real form validation (blocks Save/Publish when required and no icon is selected).
• Uses the form-control mixin so the picker participates in Umbraco’s validation lifecycle.

Changes

• Extends UmbFormControlMixin<string, typeof UmbLitElement, undefined>.
• Adds a validator in firstUpdated: this.addValidator('valueMissing', () => 'Icon is required', () => this.mandatory && !this._icon);
• Uses super.value as the single source of truth in the value getter/setter.
• Parses _icon/_color from value for rendering.
• Passes showEmptyOption: !this.mandatory to the icon picker modal to control the “empty” tile.

[nielslyngsoe]

works well, thanks!