Block Editor: Avoid default block insertion if no default block type

[aduth]

Previously: #15543 (comment)
Cherry-picks 2f1a82f from #15543 (temporarily)

This pull request seeks to resolve an error which occurs when deleting the last of the blocks in a post, if there is no default block type.

Implementation Notes:

getDefaultBlockName is implemented to handle unregistrations, and thus we should be consistent to respect this nullable return type.

gutenberg/packages/blocks/src/store/selectors.js

Line 76 in d15c484

* @return {string?} Default block name.

gutenberg/packages/blocks/src/store/reducer.js

Lines 110 to 114 in d15c484

	case 'REMOVE_BLOCK_TYPES':
	if ( action.names.indexOf( state ) !== -1 ) {
	return null;
	}
	return state;

Testing Instructions:

Verify no error when deleting all blocks if the default block type is disabled:

1. (Prerequisite) Remove the default block type:
   • wp.data.dispatch( 'core/blocks' ).removeBlockTypes( wp.data.select( 'core/blocks' ).getDefaultBlockName() );
2. Add an Image block
3. Delete the Image block
4. Ensure no errors are shown in the console