update notnull default to false

webmagic-extension/src/main/java/us/codecraft/webmagic/model/annotation/ExtractBy.java

@@ -20,7 +20,7 @@ public @interface ExtractBy {
 
     Type type() default Type.XPath;
 
-    boolean notNull() default true;
+    boolean notNull() default false;
 
     boolean multi() default false;
 

webmagic-extension/src/main/java/us/codecraft/webmagic/model/annotation/ExtractByRaw.java

@@ -20,7 +20,7 @@ public @interface ExtractByRaw {
 
     Type type() default Type.XPath;
 
-    boolean notNull() default true;
+    boolean notNull() default false;
 
     boolean multi() default false;
 

webmagic-extension/src/main/java/us/codecraft/webmagic/model/annotation/ExtractByUrl.java

@@ -16,7 +16,7 @@ public @interface ExtractByUrl{
 
     String value() default "";
 
-    boolean notNull() default true;
+    boolean notNull() default false;
 
     boolean multi() default false;