The TaskMacroConverter can throw NPE if the content of a task imported from confluence does not exist

When importing a confluence page that has a task that looks like this (missing content):
```
<ac:task>
<ac:task-id>41</ac:task-id>
<ac:task-status>incomplete</ac:task-status>
</ac:task>
```
our converter will throw a NPE because it assumes the content is not null and performs an operation over it.