Fix overwrite default value image/file with NULL

[AntonEvers]

If you set a default value for a config field type image/file and you save it. It won't set the default value in the database. Instead it will save NULL and so overwrite the default config.

With this change the default value will be set as value.

Description

This is a replication of #5085

Fixed Issues (if relevant)

1. Default values for system config of type image/file is not working #10252: Default values for system config of type image/file is not working

Manual testing scenarios

1. Install Magento from develop branch.
2. Add config field with type image.
3. Add default config value for that field.
4. Save the config field without uploading a new file.

Before the fix:

• New record created in the database table core_config_data with value NULL.

After the fix:

• New record created in the database table core_config_data with default value.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[miguelbalparda]

Hi @ajpevers I'll be reviewing this PR. Thanks for your contribution!

[miguelbalparda]

Thanks for your contribution @ajpevers!