Correct Alert Handling for Missing Compression Methods

Fixes #7940: Updated the compression check logic to improve protocol compliance. The code now returns `SSL_AD_DECODE_ERROR` when no compression method is provided in the ClientHello message. It returns `SSL_AD_ILLEGAL_PARAMETER` if the “null” compression method (0x00) is missing.

Additionally, refactored the related test code for enhanced readability and maintainability.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25255)