WP_User_Query can generate negative offset in the LIMIT clause

[birgire]

The paged argument in WP_User_Query was introduced in #34531

There's a problem with the paged as 0 case.

There's a similar ticket for WP_Comment_Query: #41826

Example:

When paged is 0 and number is 2, WP_User_Query generates the LIMIT clause:

LIMIT -2, 2 

instead of

LIMIT 0, 2 

According to the MySQL documentation, the offset in the LIMIT clause shouldn't be negative:

​https://dev.mysql.com/doc/refman/5.7/en/select.html

I think we should handle it in the same way as in WP_Query, where the 0 case is the same as 1.

[birgire]

Here we focus on the calculated offset in the LIMIT clause.

The pagination setup is made similar to the one in the other query classes.

Included is a test for this case.

@SergeyBiryukov Thanks for fixing the typo in the description