[Mappings editor] Validation error when switching between tabs

[alisonelizabeth]

Steps to reproduce:

1. Create an index template
2. Go to the "Mappings" step
3. Click on the "Dynamic templates" tab, but do not provide a value.
4. Click the "Advanced options" tab
5. Go back to the "Dynamic templates" tab. Note validation error, even though the user has provided no value.

[elasticmachine]

Pinging @elastic/es-ui (Team:Elasticsearch UI)