Filter out nulls when fetching email recipients from a group

When the 'Send a custom email' script is going through the members of a group to find their email addresses, it deliberately produces null for any inactive users to ensure that they do not receive the mail. However, such values need to be filtered out of the recipients list before constructing the message, because Jira's mail system does not handle them gracefully and will instead entirely refuse to send the email.

Root cause: inactive users are handled as "null" emails and this is causing the issue.

Fix: inactive users are not considered in the mail list.