dev/core#1339 Dedupe screen check to carry across any data where the …

…other contact has none by default.

This change in default means that if the contact to be deleted has data in a field and the contact to be kept does not the
checkbox loads as checked by default (so it's easier to keep too much data but harder to lose some.)

https://lab.civicrm.org/dev/core/issues/1339

The user can override by unchecking - this is just what is set on screen load

CRM/Contact/Form/Merge.php

@@ -225,13 +225,16 @@ public function preProcess() {
           // @todo consider enabling if it is an add & defaulting to true.
           $element[4] = array_merge((array) CRM_Utils_Array::value(4, $element, []), ['disabled' => TRUE]);
         }
-        $this->addElement($element[0],
+        $newCheckBox = $this->addElement($element[0],
           $element[1],
           array_key_exists('2', $element) ? $element[2] : NULL,
           array_key_exists('3', $element) ? $element[3] : NULL,
           array_key_exists('4', $element) ? $element[4] : NULL,
           array_key_exists('5', $element) ? $element[5] : NULL
         );
+        if (!empty($element['is_checked'])) {
+          $newCheckBox->setChecked(TRUE);
+        }
       }
 
       // add related table elements

CRM/Dedupe/Merger.php

@@ -1109,12 +1109,13 @@ public static function getRowsElementsAndInfo($mainId, $otherId, $checkPermissio
       // Display a checkbox to migrate, only if the values are different
       if ($value != $main[$field]) {
         $elements[] = [
-          'advcheckbox',
-          "move_$field",
-          NULL,
-          NULL,
-          NULL,
-          $value,
+          0 => 'advcheckbox',
+          1 => "move_$field",
+          2 => NULL,
+          3 => NULL,
+          4 => NULL,
+          5 => $value,
+          'is_checked' => (!isset($main[$field]) || $main[$field] === ''),
         ];
       }