Ensure _placeholder presence is encouraged and validated

When developing a component in tacokit, if we don't specifiy the resource's placeholder, the framework return the resource's name (a technical name).

Ie for the resource:
CsvConfiguration.customEncoding._displayName=Custom Encoding
If there's no CsvConfiguration.customEncoding._placeholder, the value displayed in the input area will be customEncoding.

Ideally, if the placeholder do not exist, the value should be an empty string.