LP 1319560: Silence template error messages in Apache error log.

LP 1319560: Silence template error messages in Apache error log.

We get a number of messages like the following in our Apache error
logs on a constant basis:

Argument "" isn't numeric in addition (+) at
/openils/var/templates/opac/parts/record/navigation.tt2 line 6.
Argument "" isn't numeric in division (/) at
/openils/var/templates/opac/results.tt2 line 1.
Argument "" isn't numeric in numeric ge (>=) at
/openils/var/templates/opac/parts/record/navigation.tt2 line 38.
Argument "" isn't numeric in numeric gt (>) at
/openils/var/templates/opac/myopac/holds/edit.tt2 line 106.
Argument "" isn't numeric in numeric gt (>) at
/openils/var/templates/opac/myopac/holds.tt2 line 42.
Argument "" isn't numeric in numeric gt (>) at
/openils/var/templates/opac/parts/place_hold.tt2 line 98.
Argument "" isn't numeric in numeric gt (>) at
/openils/var/templates/opac/parts/record/summary.tt2 line 56.
Argument "" isn't numeric in numeric gt (>) at
/openils/var/templates/opac/parts/result/table.tt2 line 87.
Argument "" isn't numeric in numeric lt (<) at
/openils/var/templates/opac/myopac/hold_history.tt2 line 122.
Argument "" isn't numeric in numeric lt (<) at
/openils/var/templates/opac/myopa/result/table.tt2 line 87.
Argument "" isn't numeric in numeric lt (<) at
/openils/var/templates/opac/myopac/hold_history.tt2 line 122.
Argument "" isn't numeric in numeric lt (<) at
/openils/var/templates/opac/myopac/holds.tt2 line 215.

This commit adds some sanity checking to make those messages go
away.

Signed-off-by: Jason Stephenson <jstephenson@mvlc.org>