fix: Fix optional fields of CorporateActionAnnouncement model

[hiohiohio]

Context:

• ValidationError for get_corporate_announcements
• CorporateActionAnnouncement should allow empty for optional fields. this causes below errors

  ValidationError: 10 validation errors for list[CorporateActionAnnouncement]
  1179.ex_date
  Field required [type=missing, input_value={'id': 'e2df04c2-c7af-4c1..., 'new_rate': '1.00015'}, input_type=dict]
  For further information visit https://errors.pydantic.dev/2.6/v/missing
  2130.target_symbol
  Field required [type=missing, input_value={'id': '8c8be13d-213d-409...', 'new_rate': '1.0175'}, input_type=dict]
  For further information visit https://errors.pydantic.dev/2.6/v/missing
  2130.target_original_cusip
  Field required [type=missing, input_value={'id': '8c8be13d-213d-409...', 'new_rate': '1.0175'}, input_type=dict]
  For further information visit https://errors.pydantic.dev/2.6/v/missing

Changes:

• set default value as None for optional fields of CorporateActionAnnouncement