Compose a module that has no resource of its own as an empty composition (issue 1945)

Its want names a composition (ADR 0539 clauses 1 and 6), so a module whose needs others meet must still have one at
its fingerprint, or no want could ever be written for it. Found by the served test.

cmd/mesh-controller/composed_writer_test.go

@@ -218,7 +218,7 @@ func TestTheCompositionsOfANodeAreItsDeclaration(t *testing.T) {
 			Resources []json.RawMessage `json:"resources"`
 		}
 		_ = json.Unmarshal(body, &d)
-		comps, moved, err := compositionsOf(node, body, fps, nil)
+		comps, moved, err := compositionsOf(node, body, fps, nil, nil)
 		if err != nil {
 			t.Fatalf("%s: %v", node, err)
 		}
@@ -253,7 +253,7 @@ func TestTheCompositionsOfANodeAreItsDeclaration(t *testing.T) {
 // unchanged it writes nothing; a changed input writes a new revision of that key alone, by compare-and-set.
 func TestACompositionIsWrittenOnceAndAgainOnlyWhenItChanges(t *testing.T) {
 	body, fps := capturedBody(t, "g14")
-	comps, _, err := compositionsOf("g14", body, fps, nil)
+	comps, _, err := compositionsOf("g14", body, fps, nil, nil)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -335,3 +335,28 @@ func TestCompositionsOutsideTheFiveNewestAreDeleted(t *testing.T) {
 		}
 	}
 }
+
+// 1945 (found by the served test): a module the node runs that composes no resource of its own still has its
+// composition, empty, at its fingerprint, so a want can name it; a module the node runs with no fingerprint of the new
+// form gets none.
+func TestAModuleWithNoResourceOfItsOwnStillHasItsComposition(t *testing.T) {
+	body := []byte(`{"declaration":1,"resources":[]}`)
+	fp := sdkdelivery.SourcePrefix2 + strings.Repeat("b", 64)
+	comps, _, err := compositionsOf("laptop", body, map[string]string{"board": fp}, nil, []string{"board", "legacy"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	key, _ := sdkdelivery.ComposedKey("laptop", "board", fp)
+	c, ok := comps[key]
+	if !ok || len(c.Resources) != 0 {
+		t.Fatalf("the board's composition: %+v (%v)", c, ok)
+	}
+	if _, err := sdkdelivery.EncodeComposed(c); err != nil {
+		t.Errorf("an empty composition does not encode: %v", err)
+	}
+	for k := range comps {
+		if strings.Contains(k, "legacy") {
+			t.Errorf("a module with no fingerprint of the new form was composed: %s", k)
+		}
+	}
+}