Fix overwriting stored email with blank string when user has no WP email

[Samwalton9]

Description

Previously, if a user did not have a Wikipedia account email, but did enter an email manually into the library, every time they logged in their email would be overwritten with a blank string. After this change, Wikipedia email data is only stored if it contains content.

Phabricator Ticket

https://phabricator.wikimedia.org/T328904

How Has This Been Tested?

Tested locally with a few different accounts having and not having email addresses linked. Added a test, which fails with the old code and passes with the new.

Types of changes

What types of changes does your code introduce? Add an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Minor change (fix a typo, add a translation tag, add section to README, etc.)

[jsnshrmn]

the code makes sense, and local testing verifies that it works. Thanks!