fix: Errors from null values. (#473 - Thanks @garemoko)

src/transformer/events/mod_assign/assignment_graded.php

src/transformer/events/mod_quiz/question_answered/essay.php

...nment_graded/existing_assignment_graded_comment/data.json

...ment_graded/existing_assignment_graded_comment/event.json

...graded/existing_assignment_graded_comment/statements.json

...gnment_graded/existing_assignment_graded_comment/test.php

...ent_graded/existing_assignment_graded_nocomment/data.json

...nt_graded/existing_assignment_graded_nocomment/event.json

...aded/existing_assignment_graded_nocomment/statements.json

...ment_graded/existing_assignment_graded_nocomment/test.php

.../mod_quiz/attempt_submitted/essay_null_response/data.json

...mod_quiz/attempt_submitted/essay_null_response/event.json

...uiz/attempt_submitted/essay_null_response/statements.json

...s/mod_quiz/attempt_submitted/essay_null_response/test.php