Redirect to login page if the mail or password fields empty.

Closes no issue

What does this MR do?

Redirect to login page if the mail or password fields empty. The redirect is after the error message.

How confident are you it won't break things if deployed?

(be honest!)

Links to related issues

https://yunity.slack.com/archives/CDFSR3Y4U/p1580505768051200

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Checkout branch locally
2. Click to login button without mail address and password
3. You should see a error message
4. Than should be a redirect to the login page

Screenshots (if applicable)

Any relevant screenshots if this is a design / frontend change

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings