missing pieces in the AST: parser seems to skip annotation attributes after a missing value

Similar to #3155, I am working on code completion based on AST nodes and came across this case for a type declaration:

```
@ConditionalOnProperty(prefix = , name = "")
```

The resulting AST does not contain any information about the `name` attribute anymore. It just gets removed from the AST, where the annotation on the type declaration looks like:

```
@ConditionalOnProperty(prefix=$missing$)
```

(using `org.eclipse.jdt.core` version 3.40.0)

(FYI @mickaelistria and @stephan-herrmann)