[BUGFIX] Field.TextViewHelper should respect global RTE Settings by default

If you enable the RTE and set defaultExtras to an empty string the resulting Behavior of the RTE
is to show everything. This commit changes sets the defaultExtras to "richtext[]" if the option 
enableRichText is set to true, but defaultExtras contains an empty string. "richtext[]" instructs
the Form to use the globally defined RTE Settings.
Could we move this to the `Text` form component's `setEnableRichText` method and have it also set `defaultExtras` if `TRUE === empty($this->defaultExtras)` (rather than comparing loosely against an empty string)? That would please my sense of tidiness ;)