chore: unified text and value properties for Topic-P (SAP#9318)

Properties types and default values of text and value of Topic-P components are now unifed.

Author: dobrinyonkov

packages/fiori/src/ShellBarItem.ts

@@ -54,7 +54,7 @@ class ShellBarItem extends UI5Element {
 	 * @public
 	 */
 	@property()
-	text?: string;
+	text = "";
 
 	/**
 	 * Defines the count displayed in the top-right corner.

packages/main/src/CheckBox.ts

@@ -194,7 +194,7 @@ class CheckBox extends UI5Element implements IFormInputElement {
 	 * @public
 	 */
 	@property()
-	text?: string;
+	text = "";
 
 	/**
 	 * Defines the value state of the component.

packages/main/src/TreeItem.ts

@@ -37,7 +37,7 @@ class TreeItem extends TreeItemBase {
 	 * @default undefined
 	 */
 	@property()
-	text?: string;
+	text = "";
 
 	/**
 	 * Defines the `additionalText`, displayed in the end of the tree item.