This is for compatibility, for people transitioning from 1.2 to 1.3.
See https://github.com/Mbed-TLS/mbedtls/issues/9223 "Mandatory server
authentication" and reports linked from there.
In the future we're likely to make server authentication mandatory in
both 1.2 and 1.3. See https://github.com/Mbed-TLS/mbedtls/issues/7080
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>