[FIX] web_editor: adds missing not null check

closes odoo/odoo#96975

X-original-commit: c2f6be56a09e3dd627922036c6deaa22aa2238dd
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

addons/web_editor/static/src/js/backend/convert_inline.js

@@ -1187,7 +1187,7 @@ function _getMatchedCSSRules(node, cssRules) {
     }
 
     for (const [key, value] of Object.entries(processedStyle)) {
-        if (value.endsWith('important')) {
+        if (value && value.endsWith('important')) {
             processedStyle[key] = value.replace(/\s*!important\s*$/, '');
         }
     };