Allow our *_gen_cleanup functions to tolerate a NULL ctx

Our *_gen_cleanup functions are essentially "free" functions. Our
free functions tolerate NULL being passed. We are being inconsistent with
our *_gen_cleanup functions. Some of them tolerate NULL and others do not.

We should consistently tolerate NULL.

See also #27795

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27813)

(cherry picked from commit 743bae4a22)

providers/implementations/keymgmt/ecx_kmgmt.c

@@ -721,6 +721,9 @@ static void ecx_gen_cleanup(void *genctx)
 {
     struct ecx_gen_ctx *gctx = genctx;
 
+    if (gctx == NULL)
+        return;
+
     OPENSSL_clear_free(gctx->dhkem_ikm, gctx->dhkem_ikmlen);
     OPENSSL_free(gctx->propq);
     OPENSSL_free(gctx);

providers/implementations/keymgmt/mac_legacy_kmgmt.c

@@ -519,6 +519,9 @@ static void mac_gen_cleanup(void *genctx)
 {
     struct mac_gen_ctx *gctx = genctx;
 
+    if (gctx == NULL)
+        return;
+
     OPENSSL_secure_clear_free(gctx->priv_key, gctx->priv_key_len);
     ossl_prov_cipher_reset(&gctx->cipher);
     OPENSSL_free(gctx);