fix: make research placeholder use same math logic as stats

[DereC4]

Description

The data displayed by the placeholder %slimefun_researches_percentage_researches_unlocked% displays data from all slimefun items (including the disabled ones) while the /sf stats displays data from all slimefun items (except the disabled ones).

Proposed changes

I'm not sure about style guide or how you wanted parity to happen, so I went with the more logical approach. Adjusted the placeholder calculation to remove disabled researches from the calculation, which matches what the /stats command does.

Made the countNonEmptyResearches public to access it from placeholder with our provided profile instance.

Related Issues (if applicable)

#4277

Checklist

• I have fully tested the proposed changes and promise that they will not break everything into chaos.
• I have also tested the proposed changes in combination with various popular addons and can confirm my changes do not break them.
• I have made sure that the proposed changes do not break compatibility across the supported Minecraft versions (1.16.* - 1.20.*).
• I followed the existing code standards and didn't mess up the formatting.
• I did my best to add documentation to any public classes or methods I added.
• I have added Nonnull and Nullable annotations to my methods to indicate their behaviour for null values
• I added sufficient Unit Tests to cover my code.

[github-actions]

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feature/**	🎈 Feature
fix/**	✨ Fix
chore/**	🧹 Chores
api/**	🔧 API
performance/**	💡 Performance Optimization
compatibility/**	🤝 Compatibility

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