refactor: use same instance for empty type vars annotation

jadx-core/src/main/java/jadx/core/dex/attributes/nodes/MethodTypeVarsAttr.java

@@ -1,19 +1,30 @@
 package jadx.core.dex.attributes.nodes;
 
+import java.util.Collections;
 import java.util.Set;
 
 import jadx.core.dex.attributes.AType;
 import jadx.core.dex.attributes.IAttribute;
 import jadx.core.dex.instructions.args.ArgType;
 
+import static jadx.core.utils.Utils.isEmpty;
+
 /**
  * Set of known type variables at current method
  */
 public class MethodTypeVarsAttr implements IAttribute {
+	private static final MethodTypeVarsAttr EMPTY = new MethodTypeVarsAttr(Collections.emptySet());
+
+	public static MethodTypeVarsAttr build(Set<ArgType> typeVars) {
+		if (isEmpty(typeVars)) {
+			return EMPTY;
+		}
+		return new MethodTypeVarsAttr(typeVars);
+	}
 
 	private final Set<ArgType> typeVars;
 
-	public MethodTypeVarsAttr(Set<ArgType> typeVars) {
+	private MethodTypeVarsAttr(Set<ArgType> typeVars) {
 		this.typeVars = typeVars;
 	}
 

jadx-core/src/main/java/jadx/core/dex/nodes/utils/TypeUtils.java

@@ -92,8 +92,9 @@ public class TypeUtils {
 			return typeVarsAttr.getTypeVars();
 		}
 		Set<ArgType> typeVars = collectKnownTypeVarsAtMethod(mth);
-		mth.addAttr(new MethodTypeVarsAttr(typeVars));
-		return typeVars;
+		MethodTypeVarsAttr varsAttr = MethodTypeVarsAttr.build(typeVars);
+		mth.addAttr(varsAttr);
+		return varsAttr.getTypeVars();
 	}
 
 	private static Set<ArgType> collectKnownTypeVarsAtMethod(MethodNode mth) {