refactor(staking): check for nil ptrs after GetCachedValue()

[raynaudoe]

Description

• Add checks for nil ptrs after calling pubkey.GetCachedValue()
• Minor code optimizations

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Summary by CodeRabbit

• New Features
  • Introduced enhanced error handling for public key validation, improving reliability when handling cached values.
  • Added a new function for validating public keys, enhancing code modularity and readability.
• Bug Fixes
  • Improved error messaging for invalid cached values to provide clearer diagnostics and prevent potential runtime errors.