add create backup is nil testcaste

Author: Benjosh95

internal/cmd/volume/backup/create/create_test.go

@@ -239,6 +239,13 @@ func TestOutputResult(t *testing.T) {
 			args:    args{},
 			wantErr: true,
 		},
+		{
+			name: "backup is nil",
+			args: args{
+				backup: nil,
+			},
+			wantErr: true,
+		},
 		{
 			name: "minimal backup",
 			args: args{