Motivation:
If the remote server returns an invalid response in the form "000 \r\n"
(i.e. a three digit code, then space, but no details), null is added
as a singletonList to the response being constructed.
This seems unexpected and it would be easier to handle an empty
details list in client code.
Modifications:
If detail is null (because frame.isReadable() returned false after
reading the separator), initialise DefaultSmtpResponse with an empty
list instead of a list containing a single null value.
Result:
When encountering this malformed server response, a DefaultSmtpResponse
with a code but no details will be created.
12a5754569