Fix fatal error when Block Compatibility is off

[xxsimoxx]

Some classes have polyfilled methods that call WP_Compat::using_block_function();.
But when Block Compatibility is set to "off" WP_Compat class is not defined.

See WS Form LITE blocks compatibility  post on the forum.

This PR adds a check for the class to exists in this way:

if ( class_exists( 'WP_Compat' ) ) {
	WP_Compat::using_block_function();
}

Docblock for WP_Compat::using_block_function has been updated to point out this.

How has this been tested?

1. Install WS Form LITE
2. Go to Posts -> All Posts
3. Click on any of the posts titles
4. No fatal error will show

Types of changes

• Bug fix

[ClassyBot]

This pull request has been mentioned on ClassicPress Forums. There might be relevant details there:

https://forums.classicpress.net/t/ws-form-lite-blocks-compatibility/5039/2