presto-main/src/main/java/io/prestosql/sql/planner/NullabilityAnalyzer.java/NullabilityAnalyzer/visitFunctionCall
class NullabilityAnalyzer:        @Override
        protected Void visitFunctionCall(FunctionCall node, AtomicBoolean result)
        {
            // TODO: this should look at whether the return type of the function is annotated with @SqlNullable
            result.set(true);
            return null;
        }