PCHR-3624: Remove warnings on Custom entity edit

[mickadoo]

Included in CiviCRM 5.2.0
Core PR: civicrm#12033

Overview

Trying to update a custom group or field worked, but it produced warnings about missing indices. This PR fixes those warnings.

Before

Updating an existing CustomGroup or CustomField would work, but produce warnings. This caused some PHPUnit tests to fail if they were updating a CustomGroup or CustomField.

For example:

$params = ['id' => 1, 'is_active' => 0];
civicrm_api3('CustomField', 'create', $params);

Will produce a warning (from a test I wrote for this):

Failure in api call for CustomGroup create:  Undefined index: extends
#0 /repos/civicrm-core/CRM/Core/BAO/CustomGroup.php(64): PHPUnit_Util_ErrorHandler::handleError(8, 'Undefined index...', '/repos/civicrm-...', 64, Array)

After

No warnings are produced.

After

I did some minor cleanup, replacing some arrays with variables and trying to shorten lines. I also deleted one unused variable, $tableName

[davialexandre]

@mickadoo Approved 👍 Please merge it using the "Rebase and merge" so that it doesn't create a new merge commit