Text.RemoveHtmlTags returns error if input HTML is null 

Text.RemoveHtmlTags returns an error if the input HTML is null. 

```
Expression.Error: We cannot convert the value null to type Text.
Details:
    Value=
    Type=Type
```

The best workaround I could come up with is to call the function wrapped in an if statement, e.g. for a Calculated Column:

`if [myHTML] = null then null else fnTextRemoveHtmlTags ( [myHTML] )`