tpl: Do not return errors in substr for out-of-bounds cases

tpl: Do not return errors in substr for out-of-bounds cases

Most other substr implementations don't error out in out-of-bounds cases
but simply return an empty string (or a value that's printed as an empty
string). We'll follow their lead and not exit template execution.  Allow
the user decide what to do with the empty result.

Fixes #8113