Fix prop-types for missingComponentComponent to allow a functional component

[vsirakova]

Description

• Add a unit test in Placeholder.test.tsx that checks the case when an unknown component is requested for rendering -> then the passed missingComponentComponent prop of the Placeholder should be used.
• Fix prop-types for missingComponentComponent prop as it currently allows only objects while it should allow a function

Motivation

When a custom MissingComponent implementation is passed to the missingComponentComponent prop there is a runtime error saying Failed prop type: Invalid prop missingComponentComponent supplied to PlaceholderComponent.
This is because the prop-types are incorrect and don't allow a function.

How Has This Been Tested?

I have added a test case that is using the missingComponentComponent prop and when this test is ran with the old prop-types then the runtime error is reproduced.
When prop-types allow for a func the runtime error disappears.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update (non-breaking change; modified files are limited to the /docs directory)

Checklist:

• I have read the Contributing guide.
• My code follows the code style of this project.
• My code/comments/docs fully adhere to the Code of Conduct.
• My change is a code change and it requires an update to the documentation.
• My change is a documentation change and it requires an update to the navigation.

[sc-illiakovalenko]

@vsirakova Thank you for contribution!