small incompatibility

[Roberto Ierusalimschy <roberto@...>]

We would like to add Mike Pall's extension on gsub (to work with tables)
still in 5.1. However, to avoid inconsistencies, it would be better
if function-call and table-lookup results were interpreted the same
way.  That means to change the meaning of functions returning nil (or
nothing) from "the replacement string is the empty string" to "makes no
replacement".

I don't think many programs have functions returning nil when they
want an empty replacement string, but anyway that would be a small
incompatibility from 5.1 alpha to beta (and final).

(Also, it seems more consistent if results different from nil/false
and string/numbers raise errors, instead of being ignored like
they are now. Again, I don't think many programs have gsubs with
functions returning weird values, but again that would be a small
incompatibility.)

-- Roberto