Call activity <all> out mappings are wrongly extracted

__Describe the Bug__

When having the following example call activity

```xml

```

[the extractor for out mappings](https://github.com/bpmn-io/extract-process-variables/blob/master/src/extractors/extractOutMappings.js) is wrongly retrieving an `undefined` variable for the `variables=all` case.


__Steps to Reproduce__

1. Create call activity with `variables=all` out mapping.
2. See the extractor having an `undefined` variable.

__Expected Behavior__

Out mappings with the form `` are ignored in the extractor.

__Environment__

- Host (Browser/Node version), if applicable: Chrome 92
- OS: macOS 11.5
- Library version: current (`0.4.3`)

Open src/extractors/extractOutMappings.js and reproduce the call activity XML containing . Verify how that form is interpreted, then confirm that extraction ignores it and no undefined variable is returned.

Written by the indexing model from the issue text.