Showcase how to handle articles is missing values on preview

[Simply007]

Motivation

According to the pull request kontent-ai#19 I am providing a showcase, how to protect articles for missing values in the preview.

1. On home page for latest articles:
   
2. On Articles section
   
3. On Article detail
   

See more details in the pull request:
kontent-ai#19

Checklist

• Code follows coding conventions held in this repo
• Automated tests have been added
• Tests are passing
• Docu has been updated (if applicable)
• Temporary settings (e.g. project ID used during development and testing) have been reverted to defaults - Please remove the preview API key and the project ID

How to test

run npm install and then npm run serve to run locally and take a look to the homepage, artiles section and article detail.

• code is hardcoded to use my modified sample project to the behavior when the values for article are not set.