Do not require Earned Credits on CourseTranscript

Current State

In Data Standard 4.0 (and probably earlier as well), the CourseTranscript requires EarnedCredits. There is no minimum value, so 0 is a allowed. The ODS/API does not enforce that the API requests include an EarnedCredit value; if the value is omitted, then the ODS/API infers a value of zero.

Proposed State

Changed EarnedCredits to be optional in the data standard, since effectively this has how it has been handled for some time now.

Reason

Improve handbook and API specification documentation to acknowledge that you don't have to supply this value. 

Avoid breaking existing implementations that are already submitting this implied 0 value.

Alternative

If we are not going to change the data standard / model / API specification, then we should ask the ODS/API to start enforcing that earnedCredits be included in payloads, instead of inferring value 0 as replacement for null.