Custom task [Output] property behavior when not set explicitly is confusing

Hello there,

I ran into a custom task property's [Output] attribute behavior which I did not expect, and which I find somewhat confusing. Also, I didn't find it documented nor discussed anywhere so here I am.

In a nutshell, if I have a custom task with a property attributed with [Output], and I don't set the value of the property in the task, MSBuild property assigned to given task property retains its previous value:
```












a;b;c

...
```
yields
```
Test:
Ids: a;b;c
Ids (after no set): a;b;c
```

Granted, _not setting_ output property value is somewhere between being unlikely and being a bug. But now that I ran into it I guess I'm spoiled by C#'s ```out``` behavior so I would expect _not setting_ would amount to target MSBuild property ending up empty. Moreover, a straightforward would-be fix
```
else
{
Identifiers = null;
}
```
is not enough, one needs
```
else
{
Identifiers = string.Empty;
}
```
to end up with what I thought I would end up with:
```
Ids: a;b;c
Ids (after no set):
```

Feels like a small trap one can fall into unexpectedly, which perhaps could be avoided by either clarifying design intent/documentation on the matter, or by cleaning property values.

Cheers,
L.