Fix a skipped character due to error in Perl script

kvg-index.json

@@ -8,6 +8,9 @@
 	".":[
 		"0002e.svg"
 	],
+	"0":[
+		"00030.svg"
+	],
 	"1":[
 		"00031.svg"
 	],

make-index.pl

@@ -14,7 +14,7 @@ my %index;
 my @files = <$Bin/kanji/*.svg>;
 for my $file (@files) {
     my ($kanji, $ex) = file_to_kanji ($file);
-    if (! $kanji) {
+    if (! defined $kanji) {
 	next;
     }
     my $tfile = $file;