Avoid loading belongsTo relations with falsey keys

[robertdp]

See #51

[robertdp]

With SQL, local keys for optional relations will most likely be nullable, which results in the query:

SELECT * FROM table WHERE id = ''

Also, the falsey check is already being used for the other relation types, just not belongsTo. Which is why only optional belongsTo relations cause unnecessary queries and Not Found! errors to be thrown.

[robertdp]

I missed handling the other half of the belongsTo logic, resulting in queries like:

SELECT * FROM table WHERE 1 = 0

Now these should be discarded without querying.

Edit: These queries were being made by the adapter before my changes, not because of them. This is probably knex translating WHERE id IN () to the equivalent and not invalid WHERE 1 = 0

[jmdobry]

Can you squash into one commit?

[robertdp]

Give me a minute

[robertdp]

Struggling with GitHub Desktop...

[robertdp]

#53