app_queue: Added initialization for "context" parameter

When using Asterisk Realtime Architecture, empty fields are skipped and the
default values are used. If the "context" parameter in queue was set and then
cleared from the database, the old value remains in memory and it continues
to be used. This change initialize the "context" parameter with an empty value,
allowing clearing the parameter.

ASTERISK-26462 #close

Change-Id: I64be73d5044ce38dd02408bd0e53de965ef65905