Convert null value in AceDoc to empty String for avoid diff errors on server and client

Fixed error on server:

```
java.lang.IllegalArgumentException: Null inputs. (patch_make)
	at name.fraser.neil.plaintext.diff_match_patch.patch_make(diff_match_patch.java:1771)
	at org.vaadin.aceeditor.ServerSideDocDiff.diff(ServerSideDocDiff.java:51)
	at org.vaadin.aceeditor.AceEditor.beforeClientResponse(AceEditor.java:270)
	at com.vaadin.server.communication.UidlWriter.write(UidlWriter.java:96)
```