onlineform: Mandatory signature fields do not validate correctly on Safari

[bobintetley]

As the title says, signature fields that have been marked mandatory don't validate and prevent submission if they are empty when the form is submitted from safari 12 (not sure if later versions are ok, needs testing).

[adamdspencer77]

Tried to test on Browser Stack. The only Safari version I saw on there was 11.1 and the signature widget doesn't seem to work at all on this version :(