Fixes for xProfile field support for WordPress Profile Fields

[needle]

This patch provides a couple of fixes for #7162:

1. Skips caching existing data for an xProfile field when it is a WordPress field
2. Fixes a minor bug where the empty item's table_name is cleared

Part 1 allows existing xProfile fields to be converted from textbox to wp-textbox format by ignoring any pre-existing values retrieved by the uncached data query.

This allows people using the ​BP XProfile WP User Sync plugin to simply change the xProfile field type and then deactivate it in favour of the new BuddyPress sync functionality. There will be orphaned field data left in the *_bp_xprofile_data table, but I think this is of little practical consequence since the field IDs are still in use.

Part 2, as the summary says, simply prevents the WordPress xProfile field table_name property from being cleared prior to priming the cache.

[needle]

Patch for #8455

[imath]

Hi @needle

Thanks a lot for your report and patch, I'll look at it asap.

[imath]

It looks good 💪. I'm going to commit it asap. Thanks again.

[imath]

In 12888:

xProfile: skip caching existing data about fields of the WP field type

This allows existing xProfile fields to be converted from the textbox to the wp-textbox type (for example) by ignoring any pre-existing values retrieved by the uncached data query.

This commit also prevents the WordPress xProfile field $table_name property from being cleared prior to priming the cache.

Props needle

Fixes #8455

[needle]

Thanks @imath