Captured body length cannot be 0

| Q | A |
| --- | --- |
| Bug? | yes |
| New Feature? | no |
| Version | 1.2.x |
#### Actual Behavior

`toolbar.captured_body_length` cannot be set  to 0 manually
#### Possible Solutions

Remove `->cannotBeEmpty()` from configuration. Maybe also use `integerNode` instead of `scalarNode`. Is that a BC break?
