Make sure the contentType of detectCharset could be null.

Author: sutra

webmagic-core/src/test/java/us/codecraft/webmagic/utils/CharsetUtilsTest.java

@@ -0,0 +1,16 @@
+package us.codecraft.webmagic.utils;
+
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+import java.io.IOException;
+
+import org.junit.jupiter.api.Test;
+
+class CharsetUtilsTest {
+
+    @Test
+    void testDetectCharset() throws IOException {
+        assertNull(CharsetUtils.detectCharset(null, new byte[0]));
+    }
+
+}