fix AfterFilter NullPointer exception in #3443

[echohlne]

I have a problem similar to #3443 and try to fix this.

fastjson version: 1.2.73(by the way, 1.7.20 works fine.)
reason:
When I create a JSONSerializer like JSONSerializer jsonSerializer = new JSONSerializer(serializeWriter, new SerializeConfig()); in my service, the class member:references will be null. Then if we custom a new AfterFilter class and try to create the json string. The AfterFilter class will throw a Nullpointer Exception like below.
exception stacktrace:

Exception in thread "main" com.alibaba.fastjson.JSONException: write javaBean error, fastjson version 1.2.73, class CustomAfterFilter$Parameter, write javaBean error, fastjson version 1.2.73, class CustomAfterFilter$ParameterDesc, fieldName : parameterDesc
	at com.alibaba.fastjson.serializer.JavaBeanSerializer.write(JavaBeanSerializer.java:539)
	at com.alibaba.fastjson.serializer.JavaBeanSerializer.write(JavaBeanSerializer.java:149)
	at com.alibaba.fastjson.serializer.JSONSerializer.write(JSONSerializer.java:312)
	at CustomAfterFilter.main(CustomAfterFilter.java:15)
Caused by: com.alibaba.fastjson.JSONException: write javaBean error, fastjson version 1.2.73, class CustomAfterFilter$ParameterDesc, fieldName : parameterDesc
	at com.alibaba.fastjson.serializer.JavaBeanSerializer.write(JavaBeanSerializer.java:539)
	at com.alibaba.fastjson.serializer.JavaBeanSerializer.write(JavaBeanSerializer.java:149)
	at com.alibaba.fastjson.serializer.FieldSerializer.writeValue(FieldSerializer.java:320)
	at com.alibaba.fastjson.serializer.JavaBeanSerializer.write(JavaBeanSerializer.java:470)
	... 3 more
Caused by: java.lang.NullPointerException
	at com.alibaba.fastjson.serializer.AfterFilter.writeKeyValue(AfterFilter.java:26)
	at CustomAfterFilter$CustomFilter.writeAfter(CustomAfterFilter.java:53)
	at com.alibaba.fastjson.serializer.AfterFilter.writeAfter(AfterFilter.java:17)
	at com.alibaba.fastjson.serializer.JavaBeanSerializer.writeAfter(JavaBeanSerializer.java:834)
	at com.alibaba.fastjson.serializer.JavaBeanSerializer.write(JavaBeanSerializer.java:502)
	... 6 more

Simplified reproduce code:

import com.alibaba.fastjson.serializer.*;

public class CustomAfterFilter {
    public static void main(String args[]) {
        SerializeWriter serializeWriter = new SerializeWriter();
        try {
            JSONSerializer jsonSerializer = new JSONSerializer(serializeWriter, new SerializeConfig());

            Parameter parameter = new Parameter();
            parameter.setParameterDesc(new ParameterDesc("vipExpireDate", "VIP expire date."));

            jsonSerializer.config(SerializerFeature.DisableCircularReferenceDetect, true);
            jsonSerializer.getAfterFilters().add(new CustomFilter());
            jsonSerializer.write(parameter);

            System.out.println(jsonSerializer.toString());
        } finally {
            serializeWriter.close();
        }
    }

    static class Parameter {
        private ParameterDesc parameterDesc;

        public ParameterDesc getParameterDesc() {
            return parameterDesc;
        }
        public void setParameterDesc(ParameterDesc parameterType) {
            this.parameterDesc = parameterType;
        }
    }

    static class ParameterDesc {
        private String parameterName;
        private String parameterUsage;
        // do some work...

        public ParameterDesc(String parameterName, String parameterUsage) {
            this.parameterName = parameterName;
            this.parameterUsage = parameterUsage;
        }
    }

    static class CustomFilter extends AfterFilter {
        @Override
        public void writeAfter(Object object) {
            if (object instanceof ParameterDesc) {
                writeKeyValue("ParameterDesc", "VIP expire date.");
            }
        }
    }
}

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[codecov-commenter]

Codecov Report

Merging #3475 into master will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master    #3475      +/-   ##
============================================
- Coverage     80.38%   80.37%   -0.02%     
+ Complexity     7472     7469       -3     
============================================
  Files           157      157              
  Lines         27912    27912              
  Branches       6731     6731              
============================================
- Hits          22438    22433       -5     
- Misses         3457     3460       +3     
- Partials       2017     2019       +2     

Impacted Files	Coverage Δ	Complexity Δ
...a/com/alibaba/fastjson/serializer/AfterFilter.java	94.73% <50.00%> (ø)	6.00 <0.00> (+1.00)
...com/alibaba/fastjson/serializer/SerialContext.java	82.69% <0.00%> (-3.85%)	21.00% <0.00%> (-1.00%)
...main/java/com/alibaba/fastjson/util/FieldInfo.java	85.95% <0.00%> (-1.01%)	96.00% <0.00%> (-3.00%)

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