Add missing Signature model JsonInclude.Include.NON_EMPTY

[andrew-m-leonard]

Adds JsonInclude.Include.NON_EMPTY to the Signature model, to prevent "null" or "" property generation.

Fixes: #271

[andrew-m-leonard]

PR resolves issue:
Signed openkeystore JSON:

{
   "bomFormat": "CycloneDX",
   "specVersion": "1.4",
   "version": 1,
   "signature": {
        "algorithm": "RS256",
        "value": "key......"
   }
}

Generated CycloneDX JSON with Signature having round-tripped JSON->Bom->JSON:

{
    "bomFormat" : "CycloneDX",
    "specVersion" : "1.4",
    "version" : 1,
    "signature" : {
        "algorithm" : "RS256",
        "value" : "key............"
    }
}