Check for star null pointer when loading constellation data

[henrysky]

Check for star null pointer when loading constellation data in case one of the major star catalog corrupted/missing, or constellation data points to HIP stars that do not exist (e.g., HIP 119000). Currently, stellarium will crash on startup when for example level 1 catalog is missing. With this PR no star catalog is needed to launch Stellarium (i.e., even level 0-3 are missing).

Another minor tweaks in this PR is search for other designations beside HIP when loading exoplanet data.

Fixes # (issue)

Screenshots (if appropriate):

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• This change requires a documentation update
• Housekeeping

How Has This Been Tested?

Test Configuration:

• Operating system: MacOS 15
• Graphics Card: Apple M3

Checklist:

• My code follows the code style of this project.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (header file)
• I have updated the respective chapter in the Stellarium User Guide
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[github-actions]

Great PR! Please pay attention to the following items before merging:

Files matching src/**/*.cpp:

• Are possibly unused includes removed?

This is an automatically generated QA checklist based on modified files.

[alex-w]

Thanks!

[github-actions]

Hello @henrysky!

Please check the fresh version (development snapshot) of Stellarium:
https://github.com/Stellarium/stellarium-data/releases/tag/weekly-snapshot

[github-actions]

Hello @henrysky!

Please check the latest stable version of Stellarium:
https://github.com/Stellarium/stellarium/releases/latest