cmd/cue: fix panic when importing empty files as a list

Empty files are represented as nil when parsed, which causes cue to
panic as the condition is not considered when asked to import as a list.

Fixes #2721

Change-Id: I20f3b49eb66661f156c06e036579ce6beddfe0d8
Signed-off-by: Thomas Way <thomas@6f.io>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1173072
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

cmd/cue/cmd/orphans.go

@@ -295,8 +295,10 @@ func placeOrphans(b *buildPlan, d *encoding.Decoder, pkg string, objs ...*ast.Fi
 			f.Decls = append(f.Decls, x.Elts...)
 		case *ast.ListLit:
 			f.Decls = append(f.Decls, &ast.EmbedDecl{Expr: x})
+		case nil:
+			f.Decls = append(f.Decls, ast.NewList())
 		default:
-			panic("unreachable")
+			panic(fmt.Sprintf("unexpected type %T", x))
 		}
 	}
 

cmd/cue/cmd/testdata/script/import_list.txtar

@@ -8,6 +8,10 @@ cmp stdout expect-stdout2
 # Issue #369
 exec cue import --with-context -l '"\(path.Ext(filename))": data' ./import3/data.json
 cmpenv import3/data.cue expect3
+
+# Issue #2721
+exec cue import -f --list issue2721/empty.yaml
+cmp issue2721/empty.cue issue2721/empty.cue.golden
 -- expect-stdout1 --
 service: [{
 	kind: "Service"
@@ -50,3 +54,7 @@ deployment: [{
 -- import3/data.json --
 [1]
 -- cue.mod --
+
+-- issue2721/empty.yaml --
+-- issue2721/empty.cue.golden --
+[]