Anonymous shapes are considered empty shapes

Given the following schema:

```
PREFIX :       <http://example.org/>

:S { :p { :q . } }
```

and data: 

```
@prefix :      <http://example.org/> .

:a      :p      :b .
:b      :q      1 , 2 .
```

The shapes map: `:a@:S` should fail but it passes with the message:

```
Node <http://example.org/b> matched empty shape
```

Example: https://tinyurl.com/y2frpzyh
