Allow optional file uploads to be nullable

[gbicann]

Currently, once a filename has been specified for an optional input parameter, there is no way to remove it.

Input parameters of type file should be changed so that they can be nulled, allowing the user to remove them by submitting a POST to the API, e.g.

POST /v1/test/{id}/inputs HTTP/1.1
content-type: application/json

{
  "epp.serverIssuedClientCertificate01": null
}

Internal ref: NRRST-1530

[gbicann]

This might be handled better in rst-api-spec where the script that exports input parameter schemas could apply the change dynamically.

[gbicann]

The parameters this would apply to are:

• epp.serverIssuedClientCertificate01
• epp.serverIssuedClientCertificate02
• srsgw.serverIssuedClientCertificate01
• srsgw.serverIssuedClientCertificate02