Fix issues in `csv_import_export_buttons.vue`

What does this MR do?

This small MR fixes the issues raised in the previous MR !57549 (comment 540826099).

Both csv_import_export_buttons.vue and csv_export_modal.vue have an issuableCount prop, but their defaults differ. Since csv_export_modal.vue is only used by csv_import_export_buttons.vue, issuableCount was changed to required: true in csv_export_modal.vue so it uses the default in the parent csv_import_export_buttons.vue. Same reasoning goes for exportCsvPath.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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Related to #326551 (closed)