evp: make all _is_a functions accept and handle a NULL argument

evp: make all _is_a functions accept and handle a NULL argument

Makes life easier for callers.

Fixes Coverity 1503326

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18799)