Change default return type from null to undefined

[54k1]

This PR changes the default result of statement_list from null to undefined.

[codecov]

Codecov Report

Merging #643 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #643   +/-   ##
=======================================
  Coverage   72.91%   72.92%           
=======================================
  Files         178      178           
  Lines       13226    13230    +4     
=======================================
+ Hits         9644     9648    +4     
  Misses       3582     3582           

Impacted Files	Coverage Δ
boa/src/exec/block/mod.rs	68.75% <100.00%> (ø)
boa/src/exec/statement_list.rs	68.75% <100.00%> (ø)
boa/src/exec/tests.rs	100.00% <100.00%> (ø)

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[Razican]

Thanks for the contribution! Could we add a link to the spec and a test for this?

[Razican]

Thanks! I think this is good to go :) We should in the future check for those "Completion" records in the spec too, which will be needed to implement async/await, and would simplify the flow for break/continue/return.