Avoid nil pointer when reading logs of a just terminated container.

Hit this when following logs of a container restarted with a health check: 
```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x168771e]

goroutine 1 [running]:
github.com/docker/compose-cli/aci.getACIContainerLogs(0x2d85260, 0xc000526c60, 0xc0001465a0, 0x24, 0xc0004f4da0, 0xa, 0xc0004f4db0, 0x7, 0x7ffeefbffab2, 0xb, ...)
	github.com/docker/compose-cli/aci/aci.go:285 +0x1fe
github.com/docker/compose-cli/aci.streamLogs(0x2d85260, 0xc000526c60, 0xc0001465a0, 0x24, 0xc0004f4da0, 0xa, 0xc0004f4db0, 0x7, 0x7ffeefbffab2, 0xb, ...)
	github.com/docker/compose-cli/aci/aci.go:297 +0x2e5
```

Signed-off-by: Guillaume Tardif <[email protected]>

aci/aci.go

@@ -282,6 +282,9 @@ func getACIContainerLogs(ctx context.Context, aciContext store.AciContext, conta
 	if err != nil {
 		return "", fmt.Errorf("cannot get container logs: %v", err)
 	}
+	if logs.Content == nil {
+		return "", nil
+	}
 	return *logs.Content, err
 }