Check attribute against value

[frame]

I have an attribute scope:

```
@PublicApi {
   // ...
}
```

Querying with hasUDA! works as expected.
Later, I added a property to PublicApi:

```
struct PublicApi {
    bool exposeDetails;
}
```

And want to ask the compiler via getUDAs! about the boolean but 
this fails with:
```Error: type PublicApi has no value```

because I need to rewrite it to

```
@PublicApi() {
   // ...
}
```

... which works again.
But how do I check against such cases? And why does a struct not 
have a value here?