Avoid throwing a warning when the birtday column is missing

When the import csv doesn't have a field called Birthday, the conversion complains about missing values.

````
Use of uninitialized value in split at csv2vcard.pl line 67, <GEN0> line 2.
Use of uninitialized value $y in sprintf at csv2vcard.pl line 68, <GEN0> line 2.
Use of uninitialized value $m in sprintf at csv2vcard.pl line 68, <GEN0> line 2.
Use of uninitialized value $d in sprintf at csv2vcard.pl line 68, <GEN0> line 2.
````