Textfield value 'true' when string emptied

[Stuart Clark]

Describe the bug
When a Textfield with a Druxt Theme component has its value emptied (add text, then delete text) the value is being set to "true"

To Reproduce
Steps to reproduce the behavior:

1. Start environment on issue branch: https://gitpod.io/#https://github.com/druxt/druxt.js/tree/feature/552-model_true_bug
2. Start DruxtSite example: yarn example:druxt-site
3. Goto contact form: /en/contact
4. Type in Name or Mail field, then delete the text.

Expected behavior

• Field should be empty, not display "true"

Media

Your Environment (please complete the following information):
N/A

Additional context
Issue appears to be caused by the bubbling of $emit'd data from the core DruxtModule component.