fix(librarian/rust): install handles nil tools (#5199)

Running `librarian install` in `google-cloud-rust` today panics b.c the
`librarian.yaml` hasn't been updated to move `release.tools.cargo` to
`tools.cargo` (see #5160).

Author: noahdietz

internal/librarian/rust/install.go

@@ -28,7 +28,7 @@ var ErrMissingToolVersion = errors.New("cargo tool missing version")
 
 // Install installs cargo tool dependencies defined in the tools configuration.
 func Install(ctx context.Context, tools *config.Tools) error {
-	if len(tools.Cargo) == 0 {
+	if tools == nil || len(tools.Cargo) == 0 {
 		return nil
 	}
 	for _, tool := range tools.Cargo {

internal/librarian/rust/install_test.go

@@ -54,7 +54,22 @@ func TestInstall_MissingVersion(t *testing.T) {
 }
 
 func TestInstall_NoCargoTools(t *testing.T) {
-	if err := Install(t.Context(), &config.Tools{}); err != nil {
-		t.Fatal(err)
+	for _, test := range []struct {
+		name  string
+		tools *config.Tools
+	}{
+		{
+			name: "nil tools",
+		},
+		{
+			name:  "no Cargo tools",
+			tools: &config.Tools{},
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			if err := Install(t.Context(), test.tools); err != nil {
+				t.Fatal(err)
+			}
+		})
 	}
 }