Fix displaying roadmap for epics without dates

What does this MR do?

It fixes the situation when a (child) epic has neither start nor due date. The problem was that due date was displayed as described in the issue (roadmap timeframe end has been taken). Now the information that epic has no start and end date is displayed.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Relates to #214026 (closed)