"Default" setting doesnt work for checkboxes

[sooskriszta]

Expected behavior: If in field setting, an option is marked as default, then it should be checked by default in frontend.

Actual behavior: Checkboxes are never checked by default.

[DJPaul]

Defaults that work:

• Radio buttons

Default that are broken:

• Checkboxes
• Multi select box
• Dropdown select box

Tested on trunk. I am going to guess these have been broken since 2.0 but we need to test on 2.1 to see if this is a regression -- if it isn't, we'll probably consider fixing for 2.3.

[slaFFik]

Some more info here #6137
But radios using xprofile_insert_field() don't work as well.

[DJPaul]

Confirmed that this broke with the rewrite in 2.0.

[DJPaul]

But radios using xprofile_insert_field() don't work as well.

Let's deal with this in #6137

[djpaul]

In 9521:

xprofile: fix broken selection of default values for checkboxes, select boxes, and multi-select boxes.

When we try to fetch a user's profile field value from the database, if it wasn't set, then we return an empty string.
A few profile field types were mishandling an empty string response from BP_XProfile_ProfileData::get_value_byid, and it was being mistakenly interpreted as a user selected value.

This change checks the return value from BP_XProfile_ProfileData::get_value_byid, and if it's empty, instead of always casting the value, sets the ongoing variable to an empty array instead.

Fixes #6134