Unassigned variables have default of null instead of undefined

[abbondanza]

The following statements both print null instead of undefined.

var x;
x;

and

let x;
x;

Testing locally, it seems like it's enough to change the defaults at https://github.com/jasonwilliams/boa/blob/8851eba27d1df86e139e101f2b0ce26dbf3a1920/src/lib/exec.rs#L395 and https://github.com/jasonwilliams/boa/blob/8851eba27d1df86e139e101f2b0ce26dbf3a1920/src/lib/exec.rs#L407

to ValueData::Undefined.

[pop]

This sounds cool and I'm interested in contributing to Boa for Hacktoberfest. Mind if I start working on this?

[abbondanza]

Go for it! Should be pretty straightforward.