Java: JavadocText does not exist for empty block comment / javadoc lines

**Description of the issue**
`JavadocText` represents a single line of a block or javadoc comment, see #3696.
However, it appears `JavadocText` does not exist for empty lines (unless they are single line comments).
Example query:
```ql
import java

from JavadocText text
where
text.getText().regexpMatch("[ \t]*")
// Make sure this is not a single line comment
and exists (text.getParent+().(Javadoc).getCommentedElement())
select text
```

It would be quite useful to detect empty lines, e.g. to find out if javadoc is missing HTML tags such as `
` or `

` to create proper line breaks. If that is the case the output will contain everything within the same line.