Align method descriptions for SslOptions getCiphers and getEnabledProtocols with '@returns'

[wilkinsona]

Both getCiphers() and getEnabledProtocols say that they return an empty set in the main description but their @returns say that they return null. Since c59c8cc, the latter is correct.

[ganjisriver]

Hi @wilkinsona Can I work on this?

[philwebb]

@ganjisriver Feel free to submit a pull-request if you'd still like to work on this one.

[snicoll]

Closing in favor of PR #46756, thanks @ganjisriver.