fix(core): if-then expression should be nullable if any 'then' is nullable

[bvolpato]

Recently had some nullability issues with Calcite coming from the SQL:

  CASE WHEN record_type = 'abc' THEN
    field_a
  ELSE
    ''
  END

Because the else part is a literal, nullable was false, but it wasn't actually true, as field_a is in fact nullable.

Calcite would fail assertions because of this mismatch:

rpc error: code = Internal desc = Cannot add expression of different type to set:
set type is RecordType(VARCHAR NOT NULL $f30, VARCHAR $f10) NOT NULL
expression type is RecordType(VARCHAR $f3, VARCHAR $f1) NOT NULL
set is org.apache.calcite.plan.volcano.RelSet@6624a103
expression is LogicalSort(sort0=[$1], dir0=[DESC], fetch=[5])
...
Type mismatch:
rowtype of original rel: RecordType(VARCHAR NOT NULL $f30, VARCHAR $f10) NOT NULL
rowtype of new rel: RecordType(VARCHAR $f3, VARCHAR $f1) NOT NULL
Difference:
$f30: VARCHAR NOT NULL -> VARCHAR

Simply allowing the case to be nullable fixes the issues for our query, so sending that upstream since I think it makes sense and it was just overlooked before.

[vbarua]

Good catch. Thanks for fixing.