Do not send empty string piece input kwarg.

[vinicvaz]

Currently if an user erase a form value leaving it blank and the field is not required we send this specific input argument as empty string.
We should avoid this behavior since it may cause divergence between the injected value into the pieces models and the expected default value.
The best way to do that is removing all not filled and not required fields from the task payload when creating a new workflow, this will help with 2 things:

• Less information in dag file.
• Ensure the models will be loaded with right default values if a field is not filled.