fix: prevent preview from being disabled when printer selected

[Jeredian]

Description

This fixes saving and showing preview setting for FLSUN-QQS printer.

Type of change

What kind of change does this PR introduce?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, local variables)
• Translations
• Documentation
• Other... Please describe:

What is the current behavior?

When user selects printer without preview image settings (simage, gimage ), preview settings become disabled.

What is the new behavior(if this is a feature change)?

• When a user selects a printer without image preview settings,
  • preview settings tab should remain enabled for the next load
• When user selects "custom" settings and set image preview settings
  • image preview settings should be saved and loaded next time
• When user selects "custom" settings, and left image preview settings empty
  • preview settings become disabled next time

How has this been tested?

Test case 1

• Open plugin settings
• Enable preview settings
• Choose FLSUN-QQS printer
• Close plugin settings dialog
• Open it again and expect that FLSUN-QQS is selected, and preview settings enabled

Test case 2

• Open plugin settings
• Enable preview settings
• Choose Custom
• Leave simage and gimage empty
• Close plugin settings dialog
• Open it again and expect that Custom is selected, and preview settings disabled

Test case 3

• Open plugin settings
• Enable preview settings
• Choose Custom
• Set simage and gimage empty
• Close plugin settings dialog
• Open it again and expect that Custom is selected, and preview settings enabled

Test case 4

• Open plugin settings
• Enable preview settings
• Choose Default
• Close plugin settings dialog
• Open it again and expect that Default is selected, and preview settings enabled

• Tested on Mac OS

Does this PR introduce a breaking change?

• Yes
• No

Does it closes one of the existing issues?

• Yes
• No

Other information

PR Checklist:

• The pull request opens from the issue/patch/feature/bugfix branch (right side) and not main branch!
• The pull request title, commit messages and code are follows our guidelines: Contribution guide
• All commits are signed-off. See How to sign-off commit
• The code changes are commented, particularly in hard-to-understand areas

[sonarcloud]

Quality Gate passed

Issues
0 New issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud