BlockBuilder.append raises an NPE when called with a ConditionalStatement

I noticed this while working on BEAM-13099. Beam's usage of BlockBuilder here raises an NPE, due to the null-checking added in https://github.com/apache/calcite/commit/d9a81b88ad561e7e4cedae93e805e0d7a53a7f1a.

We pass a ConditionalStatement to BlockBuilder.append, which doesn't match either if statement, and produces a null result.