Improve test coverage for multi-column MCV lists

The regression tests for extended statistics were not testing a couple
of important cases for the MCV lists:

  * IS NOT NULL clauses - We did have queries with IS NULL clauses, but
    not the negative case.

  * clauses with variable on the right - All the clauses had the Var on
    the left, i.e. (Var op Const), so this adds (Const op Var) too.

  * columns with fixed-length types passed by reference - All columns
    were using either by-value or varlena types, so add a test with
    UUID columns too. This matters for (de)serialization.

  * NULL-only dimension - When one of the columns contains only NULL
    values, we treat it a a special case during (de)serialization.

  * arrays containing NULL - When the constant parameter contains NULL
    value, we need to handle it correctly during estimation, for all
    IN, ANY and ALL clauses.

Discussion: https://www.postgresql.org/message-id/flat/20200113230008.g67iyk4cs3xbnjju@development
Author: Tomas Vondra

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