E_ALL Compliance: Fix PHP Notices in release.inc

release.inc produces various PHP Notices with E_ALL on. These are all due to variables, array keys or object fields which may not exist when used.

This patch fixes those that I spotted, by changing to equivalent behaviour without Notices.

Exception: Example 4 below changed to remove unwanted database query when creating a new release (and $rid->nid is not set).

Examples:

1.

  switch ($_POST['op'] ? $_POST['op'] : arg(3)) {

changed to

  switch (!empty($_POST['op']) ? $_POST['op'] : arg(3)) {

2.

    '#default_value' => $release->version,

changed to

    '#default_value' => isset($release->version) ? $release->version : NULL,

3. Added

  $output = '';

in project_release_view() and project_release_list(), that may use it later.

4. In function project_release_load() changed

  if ($rid->type == 'project_project') {
     ...
     $result = db_query("SELECT rid,version FROM {project_releases} 
                                WHERE nid = %d $status ORDER BY version DESC", $rid->nid);

to

  if ($rid->type == 'project_project' && !empty($rid->nid)) {
     ...
     $result = db_query("SELECT rid,version FROM {project_releases} 
                                WHERE nid = %d $status ORDER BY version DESC", $rid->nid);

since $rid->nid is not set on creation.