speller: do an internal spell check when --speller is an empty string

This fixes https://savannah.gnu.org/bugs/?59855.
Indirectly-reported-by: Mike Frysinger <vapier@gentoo.org>

Bug existed since version 2.9.6, commit 94347f08.

src/text.c

@@ -2533,7 +2533,7 @@ void do_spell(void)
 
 	blank_bottombars();
 
-	if (alt_speller)
+	if (alt_speller && *alt_speller)
 		treat(temp_name, alt_speller, TRUE);
 	else
 		do_int_speller(temp_name);