jsTodoTxt.parse breaking change

jsTodoTxt.parse will no longer raise an exception when the text of a task is empty.

This change is to allow behavior such as the following,

Empty tasks could be useful when combined with "hidden" tag. For example, one can create a task like this:
h:1 +project1 +project2 +project3 +project4
to define a list of projects so the app can do autocompletion for them.

Thanks to @xuhcc