Fixed a bug concerning empty fields. Functions attached to fields are no longer ignored when the field has no value.

E.g. Consider the following line:

300---<datafield tag="300" ind1=" " ind2=" "><subfield code="a"><:Num::Num:><:Num::Num::IF(,mult. p, p):></subfield></datafield>

If there is no value for the field "Num", the desirable output is:

<datafield tag="300" ind1=" " ind2=" "><subfield code="a">mult. p</subfield></datafield>

However, this did not work - when Num had no value, the function (IF in this case) was ignored and the line was not printed.
This has been fixed. Also now, given the following template line:

300---<datafield tag="300" ind1=" " ind2=" "><subfield code="a"><:Num::Num:><:Num::Num::IF(,, p):></subfield></datafield>

If the value of Num is "33", the following will be printed:

<datafield tag="300" ind1=" " ind2=" "><subfield code="a">33 p</subfield></datafield>

If however Num has no value, the line will not be printed. This is because the "IF" function will return nothing, and it is considered
undesirable to print the line.