libpq: If ALPN is not used, make PQsslAttribute(conn, "alpn") == ""

The documentation says that PQsslAttribute(conn, "alpn") returns an
empty string if ALPN is not used, but the code actually returned
NULL. Fix the code to match the documentation.

Reported-by: Michael Paquier
Discussion: https://www.postgresql.org/message-id/ZideNHji0G4gxmc3@paquier.xyz

src/bin/psql/command.c

@@ -3896,7 +3896,7 @@ printSSLInfo(void)
 		   protocol ? protocol : _("unknown"),
 		   cipher ? cipher : _("unknown"),
 		   (compression && strcmp(compression, "off") != 0) ? _("on") : _("off"),
-		   alpn ? alpn : _("none"));
+		   (alpn && alpn[0] != '\0') ? alpn : _("none"));
 }
 
 /*

src/interfaces/libpq/fe-secure-openssl.c

@@ -1837,7 +1837,7 @@ PQsslAttribute(PGconn *conn, const char *attribute_name)
 
 		SSL_get0_alpn_selected(conn->ssl, &data, &len);
 		if (data == NULL || len == 0 || len > sizeof(alpn_str) - 1)
-			return NULL;
+			return "";
 		memcpy(alpn_str, data, len);
 		alpn_str[len] = 0;
 		return alpn_str;