fix(launcher): guard null config in app provider query

[AdityaKr015]

Summary

AppProvider::query() read a config setting through a pointer that can be null. When it is null, it crashed. The fix checks for null first.

Motivation

38/82 noctalia:desktop_entry_localization FAIL 0.21s killed by signal 11 SIGSEGV

since the launcher desktop-actions toggle (#3772). Made the launcher read show_app_actions through
ConfigService*which can be null without null check. The desktop_entry_localization test builds the provider
without a config, so it segfaulted. Now a missing config just falls back to the default instead of crashing. Everything else is unchanged.

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Build / packaging

Testing

• Ran just format and just test, this time all the test passed

Manual Coverage

Not applicable

• Tested on Niri
• Tested on Hyprland
• Tested on Sway
• Tested on another compositor:
• Tested with different bar positions and density settings
• Tested at different interface scaling values
• Tested with multiple monitors

Screenshots / Videos

Just showing all test pass now

Checklist

• This PR is ready for review.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format with clang-format v22+ installed
• I ran the relevant build or test commands
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I will update end-user documentation after merge, or this PR does not change user-facing configuration or behavior.
• I added or updated assets/translations/en.json, or this PR adds no new user-facing strings.
• I did not edit non-English translation files unless this PR is explicitly for translation tooling, an import/export sync, or a maintainer-requested locale change.
• I used the existing canonical names for config keys, IPC names, paths, and identifiers.