Fixed dead code.

Fixed dead code.

While reviewing code I noticed some dead code. It was not possible to
default to the channel variable because the parameter could not be
both full and empty.

If the parameter is not a non zero length string then the code looked
like it was intending to default to the channel variable
'presence_data_cols'. If neither of these are the case it is a noop.

By enabling the dead code, you now have access to set the
'presence_data_cols' in the dialplan or scripts like lua.