ICU-23076 Add gmtime null handler

[wooffie]

Checklist

• Required: Issue filed: ICU-23076
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-23076 Add gmtime null handler"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-23076 Add gmtime null handler"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[wooffie]

Use exit because some functions in this file can call it too. Maybe we can use "default" year, but better to say user about problem in his system