Don't propagate the min range from the empty string when adding strings.

Code that triggered the bug:

  string res = "";
  foreach(a, string segment) {
    res += segment + "/";  // Adds "" + segment + "/".
    // res here got min set to 0 (kept from "").
    // ...
  }

This caused string_has_null() to return true for the above string res.

Fixes [bug 7826].