Merge pull request #272 from andrew-m-leonard/signnulls

Add missing Signature model JsonInclude.Include.NON_EMPTY

Author: stevespringett

src/main/java/org/cyclonedx/model/Signature.java

@@ -20,6 +20,7 @@
 
 import java.util.List;
 
+import com.fasterxml.jackson.annotation.JsonInclude;
 import com.fasterxml.jackson.annotation.JsonAlias;
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import com.fasterxml.jackson.annotation.JsonProperty;
@@ -28,6 +29,7 @@
 /**
  * @since 6.0.0
  */
+@JsonInclude(JsonInclude.Include.NON_EMPTY)
 @JsonIgnoreProperties(ignoreUnknown = true)
 @JsonPropertyOrder({
     "signers",