Bug 28168: leave the field empty if amount variable is empty

Bug 28168: leave the field empty if amount variable is empty

Add "if" check to the template so that it uses the value of
"debit_type.default_amount" only if it's true, i.e. not null or 0.

The reason for this patch is there's requirement from librarians -
to have this field completely empty if it's 0, so they could paste the
amount (as they usually do) without the need to clear the field first.
And anyway if you try to save the form with 0.00 value,
it won't accept it saying "Debit amount passed is not positive" so
in my opinion there's no point to preset it with zeroes to begin with.

To reproduce:
    1) head to /cgi-bin/koha/members/maninvoice.pl?borrowernumber=XXXX
    and check that field "Amount" is prefilled with 0.00;
    2) apply patch;
    3) refresh page and ensure that field "Amount" is empty now;
    4) go to /cgi-bin/koha/admin/debit_types.pl and change default
    amount to some decimal amount;
    5) refresh manual invoice page again and ensure that "Amount"
    field is prefilled with that exact decimal number;

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>