[CALCITE-6555] RelBuilder.aggregateRex wrongly thinks aggregate functions of "GROUP BY ()" queries are NOT NULL

In RelBuilder, the aggregateRex method (added in CALCITE-5802)
wrongly thinks that aggregate functions in a `GROUP BY ()`
query are NOT NULL. Consider the query

  SELECT SUM(empno) AS s, COUNT(empno) AS c
  FROM emp
  GROUP BY ()

`SUM(empno)` should be nullable, even though `empno` has type
`SMALLINT NOT NULL`, because `GROUP BY ()` will return one row
even if `emp` has no rows, and therefore `SUM` will be
evaluated over the empty set. A RelBuilder test that attempts
to build an equivalent query gets the following error stack:

  java.lang.AssertionError: type mismatch:
  ref:
  SMALLINT NOT NULL
  input:
  SMALLINT

We add a test case for measure queries, because measures are
the only code path that uses `aggregateRex` at present.