Can not create offer when startedAt field was touched but remain empty

[juliaanholcer]

Description:

Response status 400 while creating a new offer, when 'Dzień rozpoczęcia' remain empty but was previously clicked for the edition. startedAt in a request is sent as an empty string and is verified as the wrong format on the backend.

Steps to reproduce:

• go to create offer form
• edit field 'Dzień rozpoczęcia'
• check 'Akcja już trwa'
• remove date from 'Dzień rozpoczęcia'
• populate all required fields and try to submit the form - a bad request can be noticed in the console.