fix joining line with empty line

when first line has more than one child, the second child fails to
insert

blots/scroll.js

@@ -1,6 +1,7 @@
 import Parchment from 'parchment';
 import Emitter from '../core/emitter';
 import Block, { BlockEmbed } from './block';
+import Break from './break';
 import Container from './container';
 import CodeBlock from '../formats/code';
 
@@ -32,7 +33,8 @@ class Scroll extends Parchment.Scroll {
       if (last instanceof CodeBlock) {
         last.deleteAt(last.length() - 1, 1);
       }
-      first.moveChildren(last, last.children.head);
+      let ref = last.children.head instanceof Break ? null : last.children.head;
+      first.moveChildren(last, ref);
       first.remove();
     }
     this.optimize();

test/unit/blots/block.js

@@ -52,6 +52,12 @@ describe('Block', function() {
     expect(scroll.domNode).toEqualHTML('<h2>HelloWorld!</h2>');
   });
 
+  it('join line with empty', function() {
+    let scroll = this.initialize(Scroll, '<p>Hello<strong>World</strong></p><p><br></p>');
+    scroll.deleteAt(10, 1);
+    expect(scroll.domNode).toEqualHTML('<p>Hello<strong>World</strong></p>');
+  });
+
   it('join empty lines', function() {
     let scroll = this.initialize(Scroll, '<h1><br></h1><p><br></p>');
     scroll.deleteAt(1, 1);