Berry add 'bytes().appendhex()'

[s-hadinger]

Description:

Berry:

• add bytes().appendhex() used in async webserver to create an hex payload without allocating a new object
• reverting Berry allow 'bytes().append(nil)' #22758 which is not needed after all, and not fortunate

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.8
• The code change is tested and works with Tasmota core ESP32 V.3.1.0.241206
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass