MDEV-9613: keyfile without any keys crashes mysqld on loading file_key_management plugin

Code was assuming that the keys file would contain at least one valid
key. This caused a Dynamic_array::at(0) call that lead to the crash.

mysql-test/suite/encryption/r/filekeys_emptyfile.result

+call mtr.add_suppression("System key id 1 is missing at");
+call mtr.add_suppression("Plugin 'file_key_management' init function returned error");
+call mtr.add_suppression("Plugin 'file_key_management' registration.*failed");
+FOUND /System key id 1 is missing at/ in mysqld.1.err
+create table t1(c1 bigint not null, b char(200))  engine=innodb encrypted=yes encryption_key_id=1;
+ERROR HY000: Can't create table `test`.`t1` (errno: 140 "Wrong create options")
+select plugin_status from information_schema.plugins
+where plugin_name = 'file_key_management';
+plugin_status
+# Test checks if opening an empty filekeys does not crash the server.

mysql-test/suite/encryption/t/filekeys_emptyfile.opt

+--loose-file-key-management-filename=$MYSQL_TEST_DIR/std_data/empty_file

mysql-test/suite/encryption/t/filekeys_emptyfile.test

+let SEARCH_PATTERN=System key id 1 is missing at;
+source filekeys_badtest.inc;
+
+--echo # Test checks if opening an empty filekeys does not crash the server.

plugin/file_key_management/parser.cc

@@ -220,7 +220,7 @@ bool Parser::parse_file(Dynamic_array<keyentry> *keys, const char *secret)
   keys->sort(sort_keys);
   my_free(buffer);
 
-  if (keys->at(0).id != 1)
+  if (keys->elements() == 0 || keys->at(0).id != 1)
   {
     report_error("System key id 1 is missing", 0);
     return 1;