Don't send empty matches to receiver of `regexp` morph function

The `regexp` morph function currently sends empty matches to its receiver. This yields unexpected morph results. For instance, with input like this:

```<cgxml:literal name="" value="184000" />```

And calling the `regexp` function like this:

```<mm:regexp match=".*" format="resource:P${0}" />```

We get both the expected `resource:P184000` result, and an unexpected `resource:P`, resulting in failing tests with misleading error messages. See included test for details.