tls: change TLSWrap::GetProtocol to fix #56935

[nathanjjohnson7]

tls: change tlsSocket.getProtocol() to return null for server sockets

I updated TLSWrap::GetProtocol to return null for server sockets.
I updated a test to ensure that the returned protocol is indeed null for
server sockets.

Fixes: #56935

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[nodejs-github-bot]

Review requested:

• @nodejs/crypto

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.12%. Comparing base (85f5a6c) to head (12ef396).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #57036      +/-   ##
==========================================
+ Coverage   89.11%   89.12%   +0.01%     
==========================================
  Files         665      665              
  Lines      193193   193206      +13     
  Branches    37212    37220       +8     
==========================================
+ Hits       172158   172194      +36     
+ Misses      13775    13757      -18     
+ Partials     7260     7255       -5     

Files with missing lines	Coverage Δ
src/crypto/crypto_tls.cc	78.21% <100.00%> (+0.04%)	⬆️

... and 29 files with indirect coverage changes

[addaleax]

Is the return value of SSL_get_version() incorrect or misleading? If not, wouldn't we want to adjust the documentation instead of the code?

[nathanjjohnson7]

On further inspection, I think you're right. It should probably just be a documentation change. That being said, if you look at the first code snippet shared in #56935, getProtocol() is returning TLSv1.3 for an unconnected server socket. Shouldn't it be returning null as long as a tls handshake has not been completed, or am I missing something?