修复ClassUtil.getTypeArgument() 获取泛型存在null问题

hutool-core/src/test/java/cn/hutool/core/util/Issue3516Test.java

@@ -0,0 +1,36 @@
+package cn.hutool.core.util;
+
+import cn.hutool.core.lang.Console;
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.lang.reflect.Type;
+import java.util.function.Function;
+
+public class Issue3516Test {
+
+	@Test
+	public void getTypeArgumentTest() {
+		final Type typeArgument = TypeUtil.getTypeArgument(Demo.class, 0);
+		Assert.assertEquals(B.class, typeArgument);
+	}
+
+	static class Demo implements A2B{
+		@Override
+		public A apply(B arg0) {
+			final A a = new A();
+			return a;
+		}
+	}
+
+	static class A {
+		private String name;
+	}
+
+	static class B {
+		private String name;
+	}
+
+	interface A2B extends Function<B, A> {
+	}
+}