Avoid loading belongsTo relations with falsey ids

[robertdp]

No description provided.

[jmdobry]

Should it be a falsey check? Or check for undefined instead?

[jmdobry]

Also, I think pull requests are supposed to go to the develop branch.

[robertdp]

With SQL, local keys for optional relations will most likely be null, not undefined, which results in the query:

SELECT * FROM table WHERE id = ''

pull requests are supposed to go to the develop branch

Ah, okay. I can make that change if necessary.

[robertdp]

Also, the falsey check is already being used for the other relation types, just not belongsTo. Which is why only optional belongsTo relations are causing Not Found! errors to be thrown unnecessarily all over my app.

[robertdp]

Moved to #52