sql: interpret constant NULLs as TEXT when creating a view

[mgartner]

CRDB is unable to create a view when there is a untyped, constant NULL column:

defaultdb> CREATE VIEW myview AS SELECT null AS c;
ERROR: value type unknown cannot be used for table columns
SQLSTATE: 42P16

Postgres assumes untyped NULLs have the TEXT type:

marcus=# CREATE VIEW myview AS SELECT null AS c;
CREATE VIEW

marcus=# SELECT c, pg_typeof(c) FROM myview;
  c   | pg_typeof
------+-----------
 NULL | text
(1 row)

Jira issue: CRDB-17399