Re-order onReplace and insertBlocksAfter for heading unstableOnSplit

Since my last commit, the behaviour of onReplace seems to have changed a little bit. This commit ensures that an empty paragraph is added before a heading if a heading is 'split' with no before text, but with some after text (e.g. if the carat is at the beginning of a heading and the user presses ENTER)

Also removed unnecessary `event.stopImmediatePropagation()`.

packages/block-library/src/heading/edit.js

@@ -52,10 +52,7 @@ export default function HeadingEdit( {
 					insertBlocksAfter ?
 						( before, after, ...blocks ) => {
 							setAttributes( { content: before } );
-							if ( ! before.text && after.text ) {
-								onReplace( createBlock( 'core/paragraph', { content: before } ) );
-							}
-							if ( after.text ) {
+							if ( after ) {
 								insertBlocksAfter( [
 									...blocks,
 									createBlock( 'core/heading', { content: after } ),
@@ -66,6 +63,9 @@ export default function HeadingEdit( {
 									createBlock( 'core/paragraph', { content: after } ),
 								] );
 							}
+							if ( ! before && after ) {
+								onReplace( createBlock( 'core/paragraph', { content: before } ) );
+							}
 						} :
 						undefined
 				}

packages/editor/src/components/rich-text/index.js

@@ -610,11 +610,6 @@ export class RichText extends Component {
 			} else if ( event.shiftKey || ! this.onSplit ) {
 				this.onChange( insertLineBreak( record ) );
 			} else {
-				// Some implementations of onSplit might also call onReplace, and
-				// calling onReplace() will destroy the editor, so it's
-				// important that we stop other handlers (e.g. ones
-				// registered by TinyMCE) from also handling this event.
-				event.stopImmediatePropagation();
 				this.splitContent();
 			}
 		}