Job offers without salary show None in both telegram and twitter notification templates

[gdalmau]

Checklist

• I have verified that that issue exists against the master branch.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the Slack instead.)
• I have reduced the issue to the simplest possible case.

Describe the bug
When a new job offer is created without salary, it has the value None by default and the template prints this value instead of an empty string (or "Not specified")

Specify the team with the correct label
TODO: Add a label in the issue, selecting the correct team that this issue belongs to.

To Reproduce
Steps to reproduce the behavior:

• Create a job offer without specifying the salary
• Check the notification on telegram where it is showing None

Expected behavior
It is expected to show either an empty string or "Not defined" (or something else?)

[github-actions]

Wow! Many thanks for your first issue