Fix nullable-to-nonnull conversion warnings in ExecuTorchModule

[FieryRobot]

Summary:

Motivation

We have a build failure because the Xcode staging toolchain (clang19) treats -Wnullable-to-nonnull-conversion as an error. NSString.UTF8String is declared as returning const char * _Nullable, but ExecuTorch's C++ Module methods expect const char * _Nonnull. The production toolchain doesn't flag this, but the staging toolchain does, blocking the affected build.

This Diff

Adds a safeUTF8String() helper that returns string.UTF8String ?: "" — falling back to an empty string if UTF8String ever returns nil. Applied to all 8 call sites in ExecuTorchModule.mm.

Alternatives considered:

• Explicit nil check + error return at each site: Safest, but adds 5+ lines of boilerplate at each of 8 sites. Overkill given UTF8String on a valid NSString essentially never returns nil.
• Cast to _Nonnull: Simpler one-liner but crashes with undefined behavior if UTF8String ever did return nil.
• #pragma clang diagnostic ignored: Suppresses the warning but doesn't fix the underlying issue.

The ?: "" approach was chosen because: it won't crash (empty string is valid), ExecuTorch will return a "method not found" error which existing error handling already surfaces, and the nil case is essentially impossible for ASCII method name strings. The only downside is a slightly misleading error message in the astronomically unlikely failure case.

Differential Revision: D104697173

[pytorch-bot]

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[meta-codesync]

@FieryRobot has exported this pull request. If you are a Meta employee, you can view the originating Diff in D104697173.

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