PHP Notices when requesting new password

[ampt]

Submitting the lost password form without $_POST['user_login'] displays the following notices:

Notice: Undefined index: user_login in /wp-login.php on line 174
Notice: Undefined index: user_login in /wp-login.php on line 179

The current logic implies that $_POST['user_login'] is defined.

Example POST data submission:

user_login&redirect_to&wp-submit=Get%20New%20Password

We can also remove $_POST['user_email'] as it doesn't seem to be used in the lost password form.

In [19056]:

Avoid warning when user_login not in POST. Props ampt. fixes #18755