Default field value.
The default value is used when an entity is created, either:
The default value is expressed as a numerically indexed array of items, each item being an array of key/value pairs matching the set of 'columns' defined by the "field schema" for the field type, as exposed in hook_field_schema(). If the number of items exceeds the cardinality of the field, extraneous items will be ignored.
This property is overlooked if the $default_value_function is non-empty.
Example for a number_integer field:
array(
array(
'value' => 1,
),
array(
'value' => 2,
),
);
Type: array
public $default_value = array();