Add missing return value for link postfixer

[msamsel]

Suggested merge commit message (convention)

Fix: Add missing return value for link postfixer. Closes ckeditor/ckeditor5#4856.

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Additional information

For example – encountered issues, assumptions you had to make, other affected tickets, etc.

[coveralls]

Coverage remained the same at 100.0% when pulling f1eb91e on t/241 into d6c45df on master.

[jodator]

I was considering adding tests for this change but I can't think of anything not artificial. Other features don't test this. Hence merging this as is.