Orcid affiliation validation checks optional start-date field as required

[floriangantner]

Describe the bug
The validation check checks affiliation.startdate as required.

DSpace/dspace-api/src/main/java/org/dspace/orcid/model/validator/impl/OrcidValidatorImpl.java

Line 140 in 47be259

if (affiliation.getStartDate() == null) {

Nevertheless according to the orcid scheme this property start-date is not required.

https://github.com/ORCID/orcid-model/blob/master/src/main/resources/common_3.0/common-3.0.xsd

To Reproduce
Steps to reproduce the behavior:

1. Create some Profile with affiliation with education and startdate
2. Connect to orcid and enable biographical settings for affiliation
3. Push affiliation to ORcid

Expected behavior
Of course it's nice to have properly and valid data, but we expect to be able to set affiliation without some startdate.

Related work
Will provide some PR next week which will comment the regarding lines.

[floriangantner]

this might be related to #73