Add null coalescing (??) operator passing arr arg to _load_item

When we are using the jam with PHP 7.3 and if $this->_content is null following:

$this->_content[$offset]

here and there works without warnings.

From PHP 7.4 when you try to do it you will have a Notice. The following example is with PHP 7.4.13:

$ php -a
Interactive mode enabled

php > $foo=null;
php > $foo['bar'];
PHP Notice:  Trying to access array offset on value of type null in php shell code on line 1
PHP Stack trace:
PHP   1. {main}() php shell code:0

To avoid the Notice I added the checks in this PR.

php > $foo['bar']??null;
php >

PHP version upgrade

Upgrade php version to ^7.1
Upgrade phpunit version to ^7

Jam_model::save error handling improve

deprecate check_insist on Jam_model::save

Remove obsolete classes

Remove obsolete left-over empty classes

These are no longer used and they extend missing classes

Add support for Jam Price present validation

Merge pull request #83 from OpenBuildings/add-present-price-validation

Add present validation for Jam Pice

Fix collection validation loop

• Fix collection validation loop on check
• Drop PHP 5.3 support

Fix @param annotations

0.5.19...0.5.20

Improve @return annotations

0.5.19

Fix Jam_Form::fields_for() @return annotation

Fixed php warning for combining numeric and non-numeric values

• Fixed php warning for combining numeric and non-numeric values.
• Added .editorconfig for style consistency.

Improve error reporting

Add name parameter to error messages.