`branch remove` command asks for what to do with child branches when there are none

[geofflamrock]

When a branch being removed from a stack has no children we still ask for what action to take: either "Move children branches to parent branch" or "Remove children branches". This should be skipped to avoid asking an unnecessary question.

Technical notes

• The relevant command handler is RemoveBranchCommandHandler

Acceptance criteria

• When a branch being removed from a stack has no children then the user should not be asked for which action they want to take for child branches.
• The user must still be able to specifically set an action via the --remove-children or --move-children-to-parent.
• Unit tests added for the command handler covering the functionality