Bug 25261: (follow-up) item.material may contain non-numerics

Bug 25261: (follow-up) item.material may contain non-numerics

Whilst QAing bug 13547 it was highlighted to me the at the 952$3 field,
and thus the item.materials field, may contain arbitrary notes about the
material rather than just numeric values. As such we need to check for
the field being defined as aposed to greater than '0'.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>