Fix string size constraints

[abutcher-gh]

Length constraints were not being applied when handling sets as the constraints object mistakenly used size rather than sizes which is what the applying code expects.

Also, at least in my latest node version, checking for length in a string threw an exception; so I changed it to an undefined check.

[markabrahams]

Again, thank you! This is also merged and published in version 3.9.4 of the npm.