core: a Join with no condition serializes to a JoinRel that ProtoRelConverter cannot read back #1383

ProtoRelConverter.newJoin decodes rel.getExpression() without checking hasExpression() (core/src/main/java/io/substrait/relation/ProtoRelConverter.java:1015), while Join.getCondition() is Optional and RelProtoConverter leaves expression unset when it is empty. A Join built without a condition therefore serializes to a JoinRel that substrait-java cannot read back:

Join join =
    Join.builder()
        .left(t1)
        .right(t2)
        .joinType(Join.JoinType.LEFT)
        .postJoinFilter(sb.bool(true))
        .build();
io.substrait.proto.Rel proto = relProtoConverter.toProto(join); // getJoin().hasExpression() == false
protoRelConverter.from(proto); // IllegalArgumentException: Unknown type: REXTYPE_NOT_SET

The spec makes the join expression required ("Required. Can be the literal True.", site/docs/relations/logical_relations.md, v0.102.0), so the encoder is producing an out-of-spec plan. Either emit literal true when the condition is empty, or make the condition non-optional. newLateralJoin guards the same field and round-trips.

Measured on main at ca8315b. Found while reviewing #1260.