"Only update existing fields" doesn't work when key is empty

[redactedscribe]

"Only update existing fields" option doesn't work if the property/key doesn't have a value.

With the option enabled, the following will not have the timestamp added after an edit:

modified: 

But this will:

modified: test

Expected: When enabled, only update files with modified:(or custom) present, regardless if it has a value or not.

Thanks.

[alangrainger]

Fixed in 1.3.2

[redactedscribe]

It works, thanks!