[IMP] auth_password_policy: show the password meter only when needed

Bug
===
The password meter is not super pretty, so we display it only when
needed.

Task-2629544

Part-of: odoo/odoo#78298

addons/auth_password_policy/static/src/css/password_field.css

@@ -14,3 +14,7 @@ meter.o_password_meter {
     /* also input has a 5px bottom margin... */
     bottom: calc(50% - 7px + 5px)
 }
+
+input + meter.o_password_meter[length="0"] {
+    display: none;
+}

addons/auth_password_policy/static/src/js/password_gauge.js

@@ -111,6 +111,7 @@ var PasswordPolicyMeter = Widget.extend({
         low: 0.5,
         high: 0.99,
         max: 1,
+        length: 0,
         value: 0,
         optimum: 1,
     },
@@ -133,6 +134,7 @@ var PasswordPolicyMeter = Widget.extend({
      * @param {String} password
      */
     update: function (password) {
+        this.el.setAttribute('length', password.length);
         this.el.value = policy.computeScore(password, this._required, this._recommended);
     }
 });