PO-9111 - change text on add offence button if no current offences

[sarahtw-cgi]

Jira link

https://tools.hmcts.net/jira/browse/PO-9111

Change description

• If all offences are removed on the 'Offences and impositions' screen. Change the button text to 'Add an offence'
• Ensure that if offences exist in the 'Offences and impositions' screen the button text still reads 'Add another offence'

Testing done

Added unit tests and did manual testing

Security Vulnerability Assessment

CVE Suppression: Are there any CVEs present in the codebase (either newly introduced or pre-existing) that are being intentionally suppressed or ignored by this commit?

• Yes
• No

Checklist

• commit messages are meaningful and follow good commit message guidelines
• README and other documentation has been updated / added (if needed)
• tests have been updated / new tests has been added (if needed)
• Does this PR introduce a breaking change
• [ ]
  No README changes needed and does not introduce a breaking change

[sarahtw-cgi]

Build currently failing on functional tests. These are failing because a couple of tests can't find the button now that the name changes when no impositions exist

[louisbriggs]

LGTM