* Add check for empty artifact.Files slice
Tests before change
```
⇶ go test ./post-processor/digitalocean-import/... -run=TestPostProcsor_extractImageArtifact
2020/08/31 13:51:25 Looking for image in artifact
--- FAIL: TestPostProcsor_extractImageArtifact (0.00s)
panic: runtime error: index out of range [0] with length 0 [recovered]
panic: runtime error: index out of range [0] with length 0
goroutine 7 [running]:
testing.tRunner.func1.1(0xfb0300, 0xc000456460)
/usr/local/go/src/testing/testing.go:940 +0x2f5
testing.tRunner.func1(0xc0003ab560)
/usr/local/go/src/testing/testing.go:943 +0x3f9
panic(0xfb0300, 0xc000456460)
/usr/local/go/src/runtime/panic.go:969 +0x166
github.com/hashicorp/packer/post-processor/digitalocean-import.extractImageArtifact(0x0, 0x0, 0x0, 0x24, 0xc000060ea0, 0x453937, 0x1431250)
/home/wilken/Development/packer/post-processor/digitalocean-import/post-processor.go:262 +0x36d
github.com/hashicorp/packer/post-processor/digitalocean-import.TestPostProcsor_extractImageArtifact(0xc0003ab560)
/home/wilken/Development/packer/post-processor/digitalocean-import/post-processor_test.go:28 +0x2b0
testing.tRunner(0xc0003ab560, 0x1077208)
/usr/local/go/src/testing/testing.go:991 +0xdc
created by testing.(*T).Run
/usr/local/go/src/testing/testing.go:1042 +0x357
FAIL github.com/hashicorp/packer/post-processor/digitalocean-import 0.009s
FAIL
```
Tests after change
```
[go-1.14.2] [1] wilken@automaton in ~/Development/packer/ on fix_9848 (ahead 1)
⇶ go test ./post-processor/digitalocean-import/... -run=TestPostProcsor_extractImageArtifact
ok github.com/hashicorp/packer/post-processor/digitalocean-import 0.006s
```
* Update to reflect review feedback
10e74961d2