Docs: Fix example in creating-dynamic-blocks.md (#10786)

* Update creating-dynamic-blocks.md

While posts are loading inside the edit method the `posts` variable is null, attempting to access `post[0]` causes an error 

"TypeError: Cannot read property '0' of null" and the block displays "This block has encountered an error and cannot be previewed." 

checking that posts has a value resolves this.

* Added spacing to improve readability

docs/blocks/creating-dynamic-blocks.md

@@ -23,7 +23,12 @@ registerBlockType( 'my-plugin/latest-post', {
 			posts: select( 'core' ).getEntityRecords( 'postType', 'post' )
 		};
 	} )( function( props ) {
-		if ( props.posts && props.posts.length === 0 ) {
+
+		if ( ! props.posts ) {
+			return "Loading...";
+		}
+
+		if ( props.posts.length === 0 ) {
 			return "No posts";
 		}
 		var className = props.className;
@@ -59,10 +64,16 @@ registerBlockType( 'my-plugin/latest-post', {
 			posts: select( 'core' ).getEntityRecords( 'postType', 'post' )
 		};
 	} )( ( { posts, className } ) => {
+
+		if ( ! posts ) {
+			return "Loading...";
+		}
+
 		if ( posts && posts.length === 0 ) {
 			return "No posts";
 		}
-		var post = posts[ 0 ];
+
+		let post = posts[ 0 ];
 
 		return <a className={ className } href={ post.link }>
 			{ post.title.rendered }