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]

The 0 case is now same as 1.

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