Let functions return varchar instead of nvarchar (implicitly) for Fabric Data Warehouse

[YorBraakman]

When running an "SELECT ... INTO" or "CTAS" query, the command fails:

CREATE TABLE TESTTABLE AS
SELECT JSON_VALUE(x, '$.test') as x
FROM (SELECT '{"test":"test"}' as x) as u

You have to wrap each JSON_VALUE function, or other functions returning an nvarchar with a CAST/CONVERT to not get this error.

Since the nvarchar data type does not exist for tables since they're always stored the same, please just convert it for us on the go.