Export single simple configuration name has no empty value, resulting in confusing UI

Problem/Motivation

The configuration form 'Export -> Single item' has no default item for simple configuration. This leads to a confusing experience:

Proposed resolution

Add the empty value '- Select -' for simple configuration as well:

Remaining tasks

1. Write a patch
2. Review

User interface changes

Export single simple configuration name has an empty value by default '- Select -'

API changes

None

Data model changes

None