`xprofile_insert_field()` `empty()` checks break booleans when updating existing field

[boonebgorges]

If you pass a 'field_id' to xprofile_insert_field() to update an existing field, it's not possible to set any true field to false. This is because all of the checks look like:

if ( ! empty( $r['foo'] ) ) {
    $this->foo = $r['foo'];
}

and false will not pass the test.

[boonebgorges]

The same applies to any field that could optionally be set to a falsey value - as when you're setting 'field_order' to 0.

[boonebgorges]

In 9711:

Improve parameter sanitization in xprofile_insert_field().

The overzealous empty() checks meant that it was impossible to set certain
values on existing fields to falsey values.

Fixes #6354.