Check null compile_specs in delegate init

[lucylq]

The FlatBuffer schema defines compile_specs as optional, but the code unconditionally dereferenced it. Add a null check so delegates without compile_specs get initialized with an empty spec list instead of crashing.

This PR was authored with the assistance of Claude.

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Pull request overview

Fixes a crash during backend delegate initialization when the FlatBuffer compile_specs field is absent (optional in schema) by treating it as an empty compile spec list.

Changes:

• Add a null check for delegate.compile_specs() before populating and sizing compile specs.
• Default to compile_specs = nullptr and num_compile_specs = 0 so backends receive an empty ArrayRef<CompileSpec> when unspecified.

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