Recommend two fields in edfi.Credentials to be nullable.

Summary of Outcomes for Data Standard v3.3-b

• Problem Addressed: Some fields in the Credential entity do not always apply for licenses, certifications, and registrations but are not nullable in the model.
  * High-Level Reasoning: There are use cases where these elements should  not be defined and, as they are not part of the identity for Credential, it is non-breaking to change the cardinality of those elements to optional.
• Changes Made: Changed CredentialFieldDescriptor, TeachingCredentialDescriptor, and GradeLevelDescriptor elements on Credential to all be optional.

Original Ticket Description:

edfi.Credentials appears to be the table for licenses and endorsements (as well as certifications, registrations, etc.)  Some fields are not applicable to all credential types, but they are NOT NULL in the table definition.
CredentialFieldDescriptorId  - 15 values including: math, science, social studies, etc.  These apply to licenses, but not to endorsements.

TeachingCredentialDescriptorId - 15 values including: professional, substitute, provisional, temporary, etc.  These apply to licenses, but not to endorsements.