Fix stories nullable text fields

[XanderZhu]

Summary

• Make story text title and text color nullable in the model/parcelling
• Hide missing story titles and use white as the default text color when text_color is absent
• Add parceling coverage for missing and present story text color

Tests

• ./gradlew :model:testDebugUnitTest --tests org.stepik.android.model.StoryTemplateTest
• ./gradlew :app:compileDebugKotlin