Add null data check for encrypt input debugging

[jgowdy-godaddy]

Summary

• Add NullDataCheck config option (JSON or ASHERAH_NULL_DATA_CHECK env var) to detect null input data during encryption
• When enabled, logs an error if the first 64 bytes of the input buffer are all null before encryption (caller passed bad data) or after encryption (buffer was zeroed during the operation)
• Proceeds with encryption regardless to avoid runtime disruption — this is diagnostic logging only

Test plan

• All existing tests pass
• Build succeeds on Go 1.24

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