75608547b4 kernel: Remove NONNULL annotation from destroy method (Alexander Wiederin)
Pull request description:
Follow-up to https://github.com/bitcoin/bitcoin/pull/33796#discussion_r2959817508
### Summary
This PR removes the `BITCOINKERNEL_ARG_NONNULL` annotation from the `btck_block_validation_state_destroy` method in the Kernel API.
### Motivation
No other *_destroy function in the Kernel API carries the NONNULL annotation. Following the convention set by free(), destroy functions should accept null pointers.
### Usage:
Before:
```c
btck_BlockValidationState* state = NULL;
btck_block_validation_state_destroy(state); // violates nonnull contract
```
After:
```c
btck_BlockValidationState* state = NULL;
btck_block_validation_state_destroy(state); // well-defined
```
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