Core API: ChildTopics accessors which don't throw

[jri]

The ChildTopics accessors (e.g. getTopic()) and convenience accessors (e.g. getLong()) should have counterparts which return null instead of throwing an exception in case no such child topic exists.

[Jörg Richter <jri@…>]

In 86c6b056a21a3ee4e5470d2c8067ddacaffb58ed/deepamehta:

Core API: optional ChildTopics? accessors (#923).

The ChildTopics accessors and convenience accessors have counterparts which return null instead of throwing an exception in case no such child topic exists.

8 new methods in ChildTopics:

// === Accessors ===

RelatedTopic getTopicOrNull(String assocDefUri);

List<RelatedTopic> getTopicsOrNull(String assocDefUri);

// === Convenience Accessors ===

String getStringOrNull(String assocDefUri);

Integer getIntOrNull(String assocDefUri);

Long getLongOrNull(String assocDefUri);

Double getDoubleOrNull(String assocDefUri);

Boolean getBooleanOrNull(String assocDefUri);

Object getObjectOrNull(String assocDefUri);

See #923.