fix: login with unexisting user lead to a white page - EXO-70765

Before this fix, when the user logs with a non existing user, a white page is displayed instead of the login page with the classic error message
This problem is due to a test when caseInsensitive is true, which set the username to a null value
This commit add an additional test to not set the username to null

Resolves Meeds-io/meeds#1826

component/web/security/src/main/java/org/exoplatform/web/login/LoginHandler.java

@@ -200,7 +200,7 @@ public String getContextPath() {
               username = realUsername;
             }
           }
-        } else if (caseInsensitive) {
+        } else if (caseInsensitive && realUsername != null) {
           username = realUsername;
         }