Fix date validation on ADIF export

[mattmelling]

#1862, #1863

If date fields are not set when ADIF export form is submitted, the dates are submitted as empty strings.

E.g. under Firefox on my system:

-----------------------------258695364640644243403098313945
Content-Disposition: form-data; name="station_profile"

2
-----------------------------258695364640644243403098313945
Content-Disposition: form-data; name="from"


-----------------------------258695364640644243403098313945
Content-Disposition: form-data; name="to"


-----------------------------258695364640644243403098313945--

Adif_data->export_custom was previously checking if $from, $to != 0 which in this case is true, but then the blank string is passed through to DateTime::createFormat and interpreted as a truthy value, which causes the error referenced above.

This now works for me when exporting all QSOs to ADIF under PHP8.

[AndreasK79]

Tested working ok here. Thanks.