Expression short-circuiting and null friendliness for operations in Ops

[lalithsuresh]

An operation of the form CHECK x = 10 OR var1 = col1, currently gets compiled down to a series of ortools operations that constructs the full expression without returning early. If x=10 but var1 is null, we still get an NPE when passing an argument to Ops.

The above operation is equivalent to WHERE x != 10 CHECK var1 = col1, which generates an if-expression that only encodes the constraints for rows that pass the x != 10 predicate. Doing so makes this safe for use cases where var1 might be null if x=10 but has a value otherwise.

It's worth considering what the behavior should be in the presence of nulls and whether we can potentially rewrite the IR.

[lalithsuresh]

Fixing #37 would help deal with this problem.