builder/amazon Fix invalid pointer issue for non SSMAgengtEnabled builds

Tests before change
```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1392ca2]

goroutine 299 [running]:
github.com/hashicorp/packer/builder/amazon/common.(*StepCreateSSMTunnel).Cleanup(0xc0003dc460, 0x4d1a4c0, 0xc0006e9800)
        /home/wilken/Development/packer/builder/amazon/common/step_create_ssm_tunnel.go:95 +0xf2
github.com/hashicorp/packer/helper/multistep.(*BasicRunner).Run(0xc0006e98f0, 0x4d408c0, 0xc00065fcc0, 0x4d1a4c0, 0xc0006e9800)
        /home/wilken/Development/packer/helper/multistep/basic_runner.go:79 +0x2c6
github.com/hashicorp/packer/builder/amazon/ebs.(*Builder).Run(0xc000726800, 0x4d408c0, 0xc00065fcc0, 0x4d5e300, 0xc0006e8d80, 0x4cc7220, 0xc000434120, 0x0, 0x0, 0x0, ...)
        /home/wilken/Development/packer/builder/amazon/ebs/builder.go:330 +0x17e2
github.com/hashicorp/packer/packer.(*CoreBuild).Run(0xc000720500, 0x4d408c0, 0xc00065fcc0, 0x4d5e180, 0xc0006fe510, 0x0, 0x0, 0x0, 0x0, 0x0)
        /home/wilken/Development/packer/packer/build.go:287 +0x7ef
github.com/hashicorp/packer/command.(*BuildCommand).RunContext.func1(0xc0004d14d0, 0xc0003dc3c0, 0xc000441500, 0xa, 0x4d5e1e0, 0xc000720500, 0x4d408c0, 0xc00065fcc0, 0x4d5e180, 0xc0006fe510, ...)
        /home/wilken/Development/packer/command/build.go:290 +0x189
created by github.com/hashicorp/packer/command.(*BuildCommand).RunContext
        /home/wilken/Development/packer/command/build.go:284 +0xd5a
FAIL    github.com/hashicorp/packer/provisioner/shell   188.335s
FAIL
```

Test After change
```
--- PASS: TestShellProvisioner (212.39s)
    --- PASS: TestShellProvisioner/testing_amazon-ebs_builder_against_shell_provisioner (212.39s)
PASS

```

builder/amazon/common/step_create_ssm_tunnel.go

@@ -89,8 +89,18 @@ func (s *StepCreateSSMTunnel) Run(ctx context.Context, state multistep.StateBag)
 
 // Cleanup terminates an active session on AWS, which in turn terminates the associated tunnel process running on the local machine.
 func (s *StepCreateSSMTunnel) Cleanup(state multistep.StateBag) {
+	if !s.SSMAgentEnabled {
+		return
+	}
+
 	ui := state.Get("ui").(packer.Ui)
 
+	if s.session == nil || s.session.SessionId == nil {
+		msg := fmt.Sprintf("Unable to find a valid session to instance %q; skipping the termination step", s.instanceId)
+		ui.Error(msg)
+		return
+	}
+
 	ssmconn := ssm.New(s.AWSSession)
 	_, err := ssmconn.TerminateSession(&ssm.TerminateSessionInput{SessionId: s.session.SessionId})
 	if err != nil {