DefaultArangoConverter fails to serialize nested Maps with null values

[nikita-i-turvo]

Affected versions: <=3.2.5

Problem: DefaultArangoConverter#writeMap throws NullPointerException, when one of the entries contains null value.
Given document with the following shape, processing will go like this:

{
  "simpleAttribute": null, // will be skipped by `DefaultArangoConverter#writeEntity`
  "nestedMap": { // will be processed with `DefaultArangoConverter#writeInternal` and `DefaultArangoConverter#writeMap`
    "key": null // processing will be attempted with `DefaultArangoConverter#writeInternal`, which can't handle `null`s
  }
}

Similar problem (unguarded call to DefaultArangoConverter#writeInternal) exists in DefaultArangoConverter#writeCollection and DefaultArangoConverter#writeArray.

Stack trace:

java.lang.NullPointerException: null
	at com.arangodb.springframework.core.convert.DefaultArangoConverter.writeInternal(DefaultArangoConverter.java:574) ~[arangodb-spring-data-3.2.5.jar:3.2.5]
	at com.arangodb.springframework.core.convert.DefaultArangoConverter.writeMap(DefaultArangoConverter.java:707) ~[arangodb-spring-data-3.2.5.jar:3.2.5]
	at com.arangodb.springframework.core.convert.DefaultArangoConverter.writeInternal(DefaultArangoConverter.java:592) ~[arangodb-spring-data-3.2.5.jar:3.2.5]
	at com.arangodb.springframework.core.convert.DefaultArangoConverter.writeProperty(DefaultArangoConverter.java:691) ~[arangodb-spring-data-3.2.5.jar:3.2.5]
	at com.arangodb.springframework.core.convert.DefaultArangoConverter.lambda$writeEntity$7(DefaultArangoConverter.java:633) ~[arangodb-spring-data-3.2.5.jar:3.2.5]
	at org.springframework.data.mapping.model.BasicPersistentEntity.doWithProperties(BasicPersistentEntity.java:355) ~[spring-data-commons-2.2.4.RELEASE.jar:2.2.4.RELEASE]
	at com.arangodb.springframework.core.convert.DefaultArangoConverter.writeEntity(DefaultArangoConverter.java:620) ~[arangodb-spring-data-3.2.5.jar:3.2.5]
	at com.arangodb.springframework.core.convert.DefaultArangoConverter.writeInternal(DefaultArangoConverter.java:605) ~[arangodb-spring-data-3.2.5.jar:3.2.5]
	at com.arangodb.springframework.core.convert.DefaultArangoConverter.write(DefaultArangoConverter.java:564) ~[arangodb-spring-data-3.2.5.jar:3.2.5]
	at com.arangodb.springframework.core.convert.DefaultArangoConverter.write(DefaultArangoConverter.java:88) ~[arangodb-spring-data-3.2.5.jar:3.2.5]
	at com.arangodb.springframework.core.convert.ArangoEntityWriter.write(ArangoEntityWriter.java:37) ~[arangodb-spring-data-3.2.5.jar:3.2.5]
	at com.arangodb.springframework.core.template.ArangoTemplate.toVPack(ArangoTemplate.java:283) ~[arangodb-spring-data-3.2.5.jar:3.2.5]
	at com.arangodb.springframework.core.template.ArangoTemplate.insert(ArangoTemplate.java:581) ~[arangodb-spring-data-3.2.5.jar:3.2.5]
        ...

Proposed solution:

1. Add a null guard and skip an entry if value is null in DefaultArangoConverter#writeMap, similar to

value = accessor.getProperty(property);
if (value != null) {
  this.writeProperty(value, sink, property);
}

block in DefaultArangoConverter#writeEntity
2. Add a null check and write nulls with DefaultArangoConverter#writeSimple and anything else with DefaultArangoConverter#writeInternal in DefaultArangoConverter#writeCollection and DefaultArangoConverter#writeArray