Fix: Perform nil check for join constraint

[jamietanna]

Closes #2729.

[andrewmbenton]

I'm not sure the effect of treating a nil Join_constraint as a no-op is "correct". I'm not well-versed in our sqlite parser and conversion code, but maybe @orisano or @kyleconroy can weigh in.

In any case you're right that we should add a new endtoend test or a new query to an existing test for this.

[kyleconroy]

He's already on it! Fixed here #2732