This is because the {} in the code is not an object literal, but an empty block.
It is parsed as:
{}; // empty block
+ []; // this result is shown in the console
Compare with ({}) + [] which yields the same results as [] + {}; in this case the parenthesis force the {} to be treated/parsed "in an expression context".
There are a bunch of duplicates on SO about this particular dual-nature of {} (as an expression or block?) but, finding them can be somewhat tricky ..
I found https://meta.stackexchange.com/questions/83911/how-do-i-search-stackoverflow-for-at-keywords-like-private-or-synthesize on Meta, and using Symbolhound the "closest" duplicates I could find resolved around questions like this (that relate to the need to add parenthesis when "eval'ing JSON") or this (where the use of constructs like "{} == false" is a syntax error).
If anyone knows a better way to search SO for this sort question, or has a link to such a duplicate handy ..
