add missing Nullable annotation for method TtlCallable.get

src/main/java/com/alibaba/ttl/TtlCallable.java

@@ -94,7 +94,8 @@ public final class TtlCallable<V> implements Callable<V>, TtlEnhanced, TtlAttach
      * @param callable input {@link Callable}
      * @return Wrapped {@link Callable}
      */
-    public static <T> TtlCallable<T> get(Callable<T> callable) {
+    @Nullable
+    public static <T> TtlCallable<T> get(@Nullable Callable<T> callable) {
         return get(callable, false);
     }
 

src/test/java/com/alibaba/ttl/TtlCallableTest.kt

@@ -31,7 +31,7 @@ class TtlCallableTest {
         val call = Call("1", ttlInstances)
 
 
-        val ttlCallable = TtlCallable.get(call)
+        val ttlCallable = TtlCallable.get(call)!!
 
         // create after new Task, won't see parent value in in task!
         createParentTtlInstancesAfterCreateChild(ttlInstances)
@@ -130,7 +130,7 @@ class TtlCallableTest {
     @Test
     fun test_get_same() {
         val call = Call("1")
-        val ttlCallable = TtlCallable.get(call)
+        val ttlCallable = TtlCallable.get(call)!!
         assertSame(call, ttlCallable.callable)
     }