ICU-23486 Improve clarity of getOperator assertion

[ianthetechie]

This clarifies the assertion condition in what used to rely on an implicit operator. This caused compilation failures on FreeBSD 15-STABLE for a while. While the strict assertion sanitizer has been reverted upstream, I think this is clearer, so I'm opening this PR.

Refer to the Jira ticket for a fuller background.

Checklist

• Required: Issue filed: ICU-23486
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable
• Approver: Feel free to merge on my behalf

[ianthetechie]

I've tried to accept the CLA several times but it seems the app is broken so I don't know if I succeeded.

[markusicu]

I've tried to accept the CLA several times but it seems the app is broken so I don't know if I succeeded.

Please try again.

[ianthetechie]

I think it actually worked the last time I guess? When I try going through the form again, after clicking through the first page, I get this at the bottom (it remembers the info from the first time last week) but there is no further button I can press.