uSync.Community.DataTypeSerializers: revived Try-pattern serializers

Repo: KevinJump/uSync

uSync.Community.DataTypeSerializers: converted SyncDataTypeSerializerBase's guid/path lookups to a Try* pattern instead of relying on empty-string/null as failure sentinels
Updated ContentPickerConfigSerializer and MediaPicker3ConfigSerializer to map startNodeId to/from a portable entity path on export/import
Updated MNTPickerConfigSerializer to map startNode.id and startNode.dynamicRoot.originKey (the latter only when present)
Added RichTextConfigSerializer to map mediaParentId, filling a gap left by uSync.Core's built-in RichTextEditorMigratingSerializer; copies the config dictionary defensively before mutating since that serializer can hand back a read-only ImmutableSortedDictionary
Relates to issue #1051, merged via PR #1052