[11.X] Fix 0 being treated as falsey when making translation replacements

[mpstenson]

Was working with a json language string formatted as:

{
     "lg_cal_fdow": "0",
}

When I ran tinker and gave it the language tag __("lg_cal_fdow") the returned value was "lg_cal_fdow" instead of 0. This was occurring because the Elvis operator ?: considers 0 to be falsely. This fix checks for null directly.

[taylorotwell]

I worry this could break applications that rely on this behavior for empty strings.