LPLP1835982 Holds grid user barcode text generator; handle null

LPLP1835982 Holds grid user barcode text generator; handle null

Adds a new text generator for the patron barcode template in the staff
catalog holds grid.  Also adds a name field to the <eg-grid-column/>
to support the text generator.

Teach the cell text generator internals to translate undefined and null
values to '' so generator authors don't have to.

Signed-off-by: Bill Erickson <berickxx@gmail.com>
Signed-off-by: Jane Sandberg <sandbej@linnbenton.edu>