Fix static sql error

[stonebuzz]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

Fixing an SQL Error When Adding a Computer Statically to a Computer Group

SQL: INSERT INTO `glpi_plugin_databaseinventory_computergroupstatics` 
(`computers_id`, `plugin_databaseinventory_computergroups_id`) 
VALUES ('2', '')
Error: Incorrect integer value: '' for column 'plugin_databaseinventory_computergroups_id' at row 1
Backtrace:
src/DBmysql.php:1376          DBmysql->doQuery()
src/CommonDBTM.php:726        DBmysql->insert()
src/CommonDBTM.php:1345       CommonDBTM->addToDB()
...tabaseinventory/front/computergroup.form.php:72  CommonDBTM->add()
public/index.php:82           require()

Cause:
The error occurs because the plugin_databaseinventory_computergroups_id value is empty.

This happens because the item variable passed to Twig is loaded from PluginDatabaseinventoryDatabaseParam instead of PluginDatabaseinventoryComputerGroup. As a result, the code tries to insert an empty string into an integer column, which SQL rejects.

Fix : #71

Screenshots (if appropriate):