Don't delete existing `sftp.json` configuration file

[tyuldashev]

Description
In case user leaves empty SFTP port in UTBot quick setup wizard, UTBot removes sftp config file even it was pre-existing.

To Reproduce
Steps to reproduce the behavior:

1. Make sure you already have .vscode/sftp.json configuration file
2. Start UTBot Quickstart wizard
3. Proceed to 'Server Setup' page
4. Leave SFTP port field empty and click 'Next' button

Actual behavior
.vscode/sftp.json file disappears

Expected behavior
Exiting sftp.json left as is, because it could be created for reason. For example user wants to control when and what uploaded to the server. It may also contain information and settings which are necessary for user.

Basically I assume that UBot should not remove anything which wasn't created by the bot itself.

[tyuldashev]

The backup with extension .old is created but I found bigger problem with current SFTP configuration - #546.