charin(), text_char() should return something else for empty input

I have been chasing Domingo Alvarez Duarte's report of funny behavior
when assigning an empty string to a "char" variable in plpgsql. What
it comes down to is that text-to-char conversion does not behave very
well for zero-length input. charin() returns a null character, leading
to the following bizarreness:

regression=# select 'z' || (''::"char") || 'q';
?column?
----------
z
(1 row)

regression=# select length('z' || (''::"char") || 'q');
length
--------
3
(1 row)

The concatenation result is 'z\0q', which doesn't print nicely :-(.

text_char() produces a completely random result, eg:

regression=# select ''::text::"char";
?column?
----------
~
(1 row)

and could even coredump in the worst case, since it tries to fetch the
first character of the text input no matter whether there is one or not.

I propose that both of these operations should return a space character
for an empty input string. This is by analogy to space-padding as you'd
get with char(1). Any objections?

regards, tom lane