[LFX-1481] Fix display of joined date in member merge dialog for unknown date

[use-tusk]

This PR addresses the issue where the joined date in the member merge dialog shows the fallback date '1970-01-01' when the joined date is unknown. The frontend code has been modified to display '-' instead of the fallback date.

The changes were made in the member-merge-suggestions-details.vue, member-model.js, and date-time-field.js files to ensure that the display is corrected throughout the relevant components.

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[joanreyero]

Linting is failing

[use-tusk]

Addressing your review. View activity logs for details.

[use-tusk]

@joanreyero I've addressed your review. Add another review if minor changes are needed. If major changes are needed, retry the issue from the Tusk app with additional context.