Sequence zero bug

[employee451]

Explanation

Setting the sequence property to the number 0 results in a created event that doesn't have SEQUENCE. The reason for this is that the library removes falsey values from the ICS file, and 0 is a falsey value.

This PR addresses the issue by removing undefined values instead of falsey values.

Code changes

• [x] upgrade package-lock.json to version 2 (simply by running npm i)
• [x] add SEQUENCE line if it isn't undefined in src/pipeline/format.js
• [x] remove undefined instead of falsey values in src/pipeline/build.js
• [x] add a test making sure a sequence of 0 works

[employee451]

@adamgibbons Are you able to take a look at this?🙏 It is blocking for my team