When only one cert, how bad is it to create empty array for http.extra_chain_cert?
Do not like the if statement, nor do I like the empty array.
Need to sleep on it...

Tested locally using 3.1.4 and got:
warning: Passing only keyword arguments to Struct#initialize will behave differently from Ruby 3.2. Please use a Hash literal like .new({k: v}) instead of .new(k: v).

Checked the keyword vs literal on 3.3.4, Yes it behaves differently.

irb(main):008> ssl_options_without_literal_hash = Faraday::SSLOptions.new(client_cert: "this is my string")
=> #<struct Faraday::SSLOptions verify=nil, verify_hostname=nil, ca_file=nil, ca_path=nil, verify_mode=nil, cert_store=nil, client_cert="this is my string", client_key=nil, certificate=nil, private_key=nil, verify_depth=nil, version=nil, min_version=nil, max_version=...
irb(main):009> ssl_options_with_literal_hash = Faraday::SSLOptions.new({client_cert: "this is my string"})
=>
...
irb(main):010> ssl_options_with_literal_hash
=>
 verify={:client_cert=>"this is my string"},
 verify_hostname=nil,
 ca_file=nil,
 ca_path=nil,
 verify_mode=nil,
 cert_store=nil,
 client_cert=nil,
 client_key=nil,
 certificate=nil,
 private_key=nil,
 verify_depth=nil,
 version=nil,
 min_version=nil,
 max_version=nil>

Looked closer at other tests.
Thanks for all the good examples.
Found a way that should be more compatible: (Well, it worked on my machine with 3.1.4 and 3.3.4 :-D )

  let(:ssl_options) do
    Faraday::SSLOptions.new.tap do |ssl_options|
      ssl_options.client_cert = cert
    end
  end