fix ssh agent authentication

treat key name to empty string if it's not been set
by prior step.

builder/amazon/common/step_run_source_instance.go

@@ -41,7 +41,10 @@ type StepRunSourceInstance struct {
 
 func (s *StepRunSourceInstance) Run(state multistep.StateBag) multistep.StepAction {
 	ec2conn := state.Get("ec2").(*ec2.EC2)
-	keyName := state.Get("keyPair").(string)
+	var keyName string
+	if name, ok := state.GetOk("keyPair"); ok {
+		keyName = name.(string)
+	}
 	securityGroupIds := aws.StringSlice(state.Get("securityGroupIds").([]string))
 	ui := state.Get("ui").(packer.Ui)