Refactor prop types to omit redundant null for null | HTMLElement types (#998)

* refactor: omit redundant `null` prop type for HTMLElements

* yarn build:lib

* test: assert updated element prop types

src/ComboBox/ComboBox.svelte

@@ -90,7 +90,7 @@
 
   /**
    * Obtain a reference to the list HTML element
-   * @type {null | HTMLDivElement}
+   * @type {HTMLDivElement}
    */
   export let listRef = null;
 

src/ContextMenu/ContextMenu.svelte

@@ -6,7 +6,7 @@
   /**
    * Specify an element or list of elements to trigger the context menu.
    * If no element is specified, the context menu applies to the entire window
-   * @type {null | HTMLElement | HTMLElement[]}
+   * @type {HTMLElement | HTMLElement[]}
    */
   export let target = null;
 

src/DataTable/ToolbarSearch.svelte

@@ -21,7 +21,7 @@
 
   /**
    * Obtain a reference to the input HTML element
-   * @type {null | HTMLInputElement}
+   * @type {HTMLInputElement}
    */
   export let ref = null;
 

src/MultiSelect/MultiSelect.svelte

@@ -136,13 +136,13 @@
 
   /**
    * Obtain a reference to the field box element
-   * @type {null | HTMLDivElement}
+   * @type {HTMLDivElement}
    */
   export let fieldRef = null;
 
   /**
    * Obtain a reference to the selection element
-   * @type {null | HTMLDivElement}
+   * @type {HTMLDivElement}
    */
   export let selectionRef = null;
 

src/TreeView/TreeViewNode.svelte

@@ -22,7 +22,7 @@
   /**
    * Finds the nearest parent tree node
    * @param {HTMLElement} node
-   * @returns {null | HTMLElement}
+   * @returns {HTMLElement}
    */
   function findParentTreeNode(node) {
     if (node.classList.contains("bx--tree-parent-node")) return node;