[BUG] NullPointer if metaData is null

Hey,

we'r getting a NPE while calling /api/admin/users if a user's metaData field is null.

authy_1  | Caused by: java.lang.NullPointerException: null
authy_1  | 	at com.fasterxml.jackson.core.JsonFactory.createParser(JsonFactory.java:889) ~[jackson-core-2.9.9.jar!/:2.9.9]
authy_1  | 	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3023) ~[jackson-databind-2.9.9.jar!/:2.9.9]
authy_1  | 	at de.reynok.authentication.core.util.HashMapConverter.convertToEntityAttribute(HashMapConverter.java:31) ~[classes!/:1.0.0-SNAPSHOT]
authy_1  | 	at de.reynok.authentication.core.util.HashMapConverter.convertToEntityAttribute(HashMapConverter.java:12) ~[classes!/:1.0.0-SNAPSHOT]
authy_1  | 	at org.hibernate.metamodel.model.convert.internal.JpaAttributeConverterImpl.toDomainValue(JpaAttributeConverterImpl.java:45) ~[hibernate-core-5.3.10.Final.jar!/:5.3.10.Final]
authy_1  | 	at org.hibernate.type.descriptor.converter.AttributeConverterSqlTypeDescriptorAdapter$2.doConversion(AttributeConverterSqlTypeDescriptorAdapter.java:140) ~[hibernate-core-5.3.10.Final.jar!/:5.3.10.Final]
authy_1  | 	... 110 common frames omitted