Fix Promise->map hang on false values after concurrency limit

[ilmari]

The check for whether to call the callback returned early if the item was falsy. Instead, it needs to only return early if there are no items left.

[jberger]

In theory this can be done without adding another line by making the later line https://github.com/mojolicious/mojo/pull/2084/files#diff-d48a9dbb018a19b463cfae82ffb2a5fd0cfe0cf4a553c09f8aba1048aaaaba0aR78 instead say for shift @$items but I don't know if that makes the line too long or too inscrutable. @kraih?

[jberger]

Either way, I approve

[ilmari]

for shift @$items

Another option would be return () unless my ($item) = splice(@items, 0, 1);, since splice returns an empty list if the input is empty, and list assignment returns the number of items assigned, regardless of their truthiness.