Fix CurseForge rejecting files without relations.

src/main/java/net/darkhax/curseforgegradle/UploadArtifact.java

@@ -469,7 +469,12 @@ private Metadata createMetadata() {
         request.changelogType = TaskPublishCurseForge.parseString(this.changelogType);
         request.displayName = TaskPublishCurseForge.parseString(this.displayName);
         request.releaseType = TaskPublishCurseForge.parseString(this.releaseType);
-        request.relations = this.uploadRelations;
+
+        // Only set the relations if they actually exist. Curse doesn't like empty arrays here :upside_down:
+        if (!this.uploadRelations.getRelations().isEmpty()) {
+
+            request.relations = this.uploadRelations;
+        }
 
         // If the parent is null this is a parent artifact.
         if (this.parent == null) {
@@ -490,7 +495,7 @@ private Metadata createMetadata() {
             // file. The curseFileId is null until the parent has been published.
             request.parentFileID = this.parent.curseFileId;
         }
-
+        
         return request;
     }
 }

src/main/java/net/darkhax/curseforgegradle/api/metadata/Metadata.java

@@ -39,5 +39,5 @@ public class Metadata {
 
     @Expose
     @SerializedName("relations") // Optional
-    public ProjectRelations relations = new ProjectRelations();
+    public ProjectRelations relations = null;
 }