This channel's associated identifier, or the empty string if it has handle type NONE.
For channels where TpChannelIface:handle is non-zero, this is the result of inspecting TpChannelIface:handle.
This is not guaranteed to be set until prepare_async has finished
preparing TP_CHANNEL_FEATURE_CORE; until then, it may be the empty string.
Changed in 0.11.4: this property is never null. Previously, it was null before an identifier was known, or when
a channel with no TargetID D-Bus property had TargetHandleType NONE.