Avoid PHP notice on 'Manage Contribution Pages' screen

[braders]

Overview

Avoid PHP notice on 'Manage Contribution Pages' screen.

Before

If no contribution pages exist, the notice Undefined index: rows occured. As shown in a WordPress environment, with query monitor catching and showing notices:

After

The rows smarty variable is always defined, defaulting to am empty array if no PCPs exist. As such the PHP notice has gone.

Additionally the variable $contribution has been pluralised for better readability.

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).

[colemanw]

Makes sense.