Fix Xcode 26 compatibility: replace NULL with nullptr in SampleOscillator

[jabou]

Summary

• Replace NULL with C++ nullptr in SampleOscillator.h
• Xcode 26.4 stricter headers no longer implicitly provide NULL via <math.h>

Test plan

• swift build compiles successfully

[NickCulbertson]

Thanks! This looks good to me! NULL is still working for me in Xcode 26, but we use nullptr in other places so seems like a good fix.

[jabou]

Hey @NickCulbertson

This affects Xcode 26.4., There were many C++ changes made by Apple in that version, so there's an issue.
My bad, the description was a bit misleading, I'll update it now.

[NickCulbertson]

@aure This PR works for me.

[getdunne]

You fellows will have to approve this. I dont' think I'm authorized anymore.

[jabou]

Hi @aure ,

Would it be possible to create a new version that includes this fix so we can update the project link to the officially released version?

Thank you!