fix: empty the webhookParameters var after sending an envelope

[WENKz]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
License	MIT

When sending a batch of documents, the token of the previous transaction is kept to regenerate the new validation token which prevents validating the docusign callback after signature

[vincentchalamon]

Thanks @WENKz