CSI: correctly handly empty capacity fields during registration

When a volume is registered via `nomad volume register` and not via the
creation workflow, it may not have any of the capacity fields set (as they're
not used for registration). Handle this case without error, and let the
downstream RPCs handle the error case for `nomad volume create`.

command/volume_register_csi.go

@@ -126,12 +126,16 @@ func parseCapacityBytes(cap *ast.ObjectList) (int64, error) {
 			if !ok {
 				break
 			}
-			b, err := humanize.ParseBytes(strings.Trim(lit.Token.Text, "\""))
+			literal := strings.Trim(lit.Token.Text, "\"")
+			if literal == "" {
+				return 0, nil
+			}
+			b, err := humanize.ParseBytes(literal)
 			if err != nil {
 				return 0, fmt.Errorf("could not parse value as bytes: %v", err)
 			}
 			return int64(b), err
 		}
 	}
-	return 0, fmt.Errorf("could not parse value as bytes")
+	return 0, nil
 }

command/volume_register_test.go

@@ -51,7 +51,7 @@ func TestCSIVolumeDecode(t *testing.T) {
 		expected *api.CSIVolume
 		err      string
 	}{{
-		name: "typical volume",
+		name: "volume creation",
 		hcl: `
 id              = "testvolume"
 name            = "test"
@@ -110,6 +110,34 @@ capability {
 			Secrets:    map[string]string{"password": "xyzzy"},
 		},
 		err: "",
+	}, {
+		name: "volume registration",
+		hcl: `
+id              = "testvolume"
+external_id     = "vol-12345"
+name            = "test"
+type            = "csi"
+plugin_id       = "myplugin"
+capacity_min    = "" # meaningless for registration
+
+capability {
+  access_mode     = "single-node-writer"
+  attachment_mode = "file-system"
+}
+`,
+		expected: &api.CSIVolume{
+			ID:         "testvolume",
+			ExternalID: "vol-12345",
+			Name:       "test",
+			PluginID:   "myplugin",
+			RequestedCapabilities: []*api.CSIVolumeCapability{
+				{
+					AccessMode:     api.CSIVolumeAccessModeSingleNodeWriter,
+					AttachmentMode: api.CSIVolumeAttachmentModeFilesystem,
+				},
+			},
+		},
+		err: "",
 	},
 	}