Allow for `null` and `empty` parameters in the MultiField annotation.

[youssef3wi]

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Closes #2952

[youssef3wi]

I think it would be better to include these parameters in https://github.com/spring-projects/spring-data-elasticsearch/blob/d079a59cb4568a206764f522bb3fd4ca44af59c5/src/main/java/org/springframework/data/elasticsearch/annotations/InnerField.java#L181

boolean storeNullValue() default false;
boolean storeEmptyValue() default true;

to also support inner fields

https://github.com/spring-projects/spring-data-elasticsearch/blob/d079a59cb4568a206764f522bb3fd4ca44af59c5/src/main/java/org/springframework/data/elasticsearch/core/mapping/SimpleElasticsearchPersistentProperty.java#L113

storeNullValue = isField ? getRequiredAnnotation(Field.class).storeNullValue() 
: isMultiField && (getRequiredAnnotation(MultiField.class).mainField().storeNullValue() 
|| Arrays.stream(getRequiredAnnotation(MultiField.class).otherFields())
.anyMatch(InnerField::storeNullValue));

storeEmptyValue = isField ? getRequiredAnnotation(Field.class).storeEmptyValue() 
: !isMultiField || getRequiredAnnotation(MultiField.class).mainField().storeEmptyValue() 
|| Arrays.stream(getRequiredAnnotation(MultiField.class).otherFields())
.anyMatch(InnerField::storeEmptyValue);

[sothawo]

I just tried to use this in a sample application, but it fails when trying to create the index with the null_value mapping in https://github.com/spring-projects/spring-data-elasticsearch/blob/main/src/main/java/org/springframework/data/elasticsearch/client/elc/RequestConverter.java#L258.

Might be an error on our side, or an error in the Elasticsearch client. Won't have any more time today to check.

We should have an integration test for this that creates an index with the mapping, stores a value and retrieves it.

[sothawo]

TIL: you cannot set a null_value on a field of type text. This fails on Elasticsearch level, not only in the Elasticsearch client library.

Although the documentation about null_value does not mention this.

Will get back to your PR tomorrow.