Fix TypeError: no implicit conversion of nil into String for Changelog

What does this MR do and why?

Contributes to #568851 (closed)

Problem

The changelog generation API endpoint fails with a TypeError when attempting to match a nil value against a regular expression in the UntrustedRegexp#match method.

Solution

Add missing validation for empty version case.

References

TypeError: no implicit conversion of nil into S... (#568851 - closed)

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Before	After

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Related to #568851 (closed)