Fix lexer: Strings can be empty

lib/Minilisp.pm

@@ -75,7 +75,7 @@ sub tokenize {
             value => $1,
           };
         }
-      elsif($str =~ s/^"(([^"\\]|\\.)+)"//)
+      elsif($str =~ s/^"(([^"\\]|\\.)*)"//)
         {
           $debug->("String '$1'");
           my $value = $1;

t/strings.t

@@ -1,7 +1,9 @@
-(plan 4)
+(plan 5)
 
 (expect "length of string" (= (length "hello") 5))
 
+(expect "empty strings" (= (length "") 0))
+
 (expect "string-upcase" (string= (string-upcase "helLo") "HELLO"))
 
 (expect "string-downcase" (string= (string-downcase "HEllO") "hello"))