Inheriting a String value in subtask creation fails if parent value is null

[Stefan Glase]

For example if the parent issue has no description the following task would fail:

- new task
  description: @inherit

Tasks

• add null check for all String replacements in SubtasksCreationService

Considerations

• Summary cannot be null (no need to check here)
• Description can be null? Or empty String?
• Custom field TEXTAREA can be null? Or empty String?
• Custom field TEXT can be null? Or empty String?