Reformat the string output to be more in line with the decisions made in #3465

Don't highlight excess when comparing unequal length strings.

Clean up the NULL / empty string display.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3514)