Fix TwitterImage: Cannot return null for non-nullable field \"MediaItem.databaseId\"."

[kuuak]

When no twitter image set, the resolver returned an empty string. This is not consistent with othe MediaItem fields, and make any non-nullable field in the MediaItem type throw an error.

As you can see on the screenshots below, for opengraphImage field return null instead of an object with properties with empty string.

opengraphImage returns null	twitterImage returns an object with empty string properties

This pull request fixes this and simply if not twitter image set, the property twitterImage will be null.

=> after the fix

[ashhitch]

@kuuak thanks for this will get it merged in