Empty Executor Config will cause invalid memory address or nil pointer dereference in SHELL Executor

[yinyajun]

Describe the bug

In a new job, I set executor=shell, but forgot to set executor_config.
When this job executes, it will cause invalid memory address or nil pointer dereference and panics dkron agent.

To Reproduce
Steps to reproduce the behavior:

1. add a new job
2. set executor=shell
3. leave executor_config blank
4. run this job

Expected behavior
Empty Executor Config in a job should do nothing and will not panics dkron agent

Bug
In dkron/builtin/bins/dkron-executor-shell/shell.go, Line 150-176:

// Determine the shell invocation based on OS
func buildCmd(command string, useShell bool, env []string, cwd string) (cmd *exec.Cmd, err error) {
	var shell, flag string

	if useShell {
		if runtime.GOOS == windows {
			shell = "cmd"
			flag = "/C"
		} else {
			shell = "/bin/sh"
			flag = "-c"
		}
		cmd = exec.Command(shell, flag, command)
	} else {
		args, err := shellwords.Parse(command)
		if err != nil {
			return nil, err
		}
		cmd = exec.Command(args[0], args[1:]...)
	}
	if env != nil {
		cmd.Env = append(os.Environ(), env...)
	}
	cmd.Dir = cwd
	return
}

if command is an empty string, then args is a empty array.

args[0] and args[1:] will out of array bounds and cause invalid memory address or nil pointer dereference.