[FIX] web_editor: traceback selection null in FF

In Firefox only :
In some case (editor in page but not visible) the
`document.getSelection` sometimes return `null` instead
of an empty selection object. We add some check to ensure
we avoid unnecessary traceback.

task-2971126

closes odoo/odoo#99511

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/web_editor/static/src/js/editor/odoo-editor/src/utils/utils.js

@@ -1128,7 +1128,7 @@ export function containsUnremovable(node) {
 
 export function getInSelection(document, selector) {
     const selection = document.getSelection();
-    const range = !!selection.rangeCount && selection.getRangeAt(0);
+    const range = selection && !!selection.rangeCount && selection.getRangeAt(0);
     return (
         range &&
         (closestElement(range.startContainer, selector) ||

addons/web_editor/static/src/js/wysiwyg/wysiwyg.js

@@ -1769,7 +1769,7 @@ const Wysiwyg = Widget.extend({
         this.toolbar.$el.find('.only_fa').toggleClass('d-none', !$target.is('.fa'));
         // Hide the create-link button if the selection spans several blocks.
         const selection = this.odooEditor.document.getSelection();
-        const range = selection.rangeCount && selection.getRangeAt(0);
+        const range = selection && selection.rangeCount && selection.getRangeAt(0);
         const $rangeContainer = range && $(range.commonAncestorContainer);
         const spansBlocks = range && !!$rangeContainer.contents().filter((i, node) => isBlock(node)).length;
         if (!range || spansBlocks) {