Fix nullpointer in ModelRenderer

- Somehow managed to only do a check in a subclass
 - Add #empty() helper method to IModel

src/main/java/com/jozufozu/flywheel/backend/model/ArrayModelRenderer.java

@@ -16,7 +16,7 @@ public class ArrayModelRenderer extends ModelRenderer {
 
 	@Override
 	public void draw() {
-		if (!isInitialized()) init();
+		if (!initialized) init();
 		if (!isValid()) return;
 
 		vao.bind();
@@ -26,16 +26,12 @@ public class ArrayModelRenderer extends ModelRenderer {
 		vao.unbind();
 	}
 
-	private boolean isValid() {
-		return model != null && model.valid();
-	}
-
 	@Override
 	protected void init() {
 		initialized = true;
 		IModel model = modelSupplier.get();
 
-		if (model.vertexCount() <= 0) return;
+		if (model.empty()) return;
 
 		this.model = new IndexedModel(model);
 

src/main/java/com/jozufozu/flywheel/backend/model/ModelRenderer.java

@@ -19,30 +19,29 @@ public class ModelRenderer {
 	 * Renders this model, checking first if there is anything to render.
 	 */
 	public void draw() {
-
-		if (!isInitialized()) init();
-		if (!model.valid()) return;
+		if (!initialized) init();
+		if (!isValid()) return;
 
 		model.setupState();
 		model.drawCall();
 		model.clearState();
 	}
 
-	protected void init() {
-		initialized = true;
-		IModel model = modelSupplier.get();
-
-		if (model.vertexCount() <= 0) return;
-
-		this.model = new IndexedModel(model);
-	}
-
-	public boolean isInitialized() {
-		return initialized;
-	}
-
 	public void delete() {
 		if (model != null)
 			model.delete();
 	}
+
+	protected void init() {
+		initialized = true;
+		IModel model = modelSupplier.get();
+
+		if (model.empty()) return;
+
+		this.model = new IndexedModel(model);
+	}
+
+	protected boolean isValid() {
+		return model != null && model.valid();
+	}
 }

src/main/java/com/jozufozu/flywheel/core/model/IModel.java

@@ -63,4 +63,12 @@ public interface IModel {
 	default int size() {
 		return vertexCount() * format().getStride();
 	}
+
+	/**
+	 * Is there nothing to render?
+	 * @return true if there are no vertices.
+	 */
+	default boolean empty() {
+		return vertexCount() == 0;
+	}
 }