fix: parse empty strings

parse_string_internal, based on nom's escaped_transform only parsed strings with at least one character. I think escaped_transform itself is buggy, but it was faster to special case empty-strings in the biscuit grammar itself.

biscuit-parser/src/parser.rs

@@ -695,7 +695,10 @@ fn parse_string_internal(i: &str) -> IResult<&str, String, Error> {
 }
 
 fn parse_string(i: &str) -> IResult<&str, String, Error> {
-    delimited(char('"'), parse_string_internal, char('"'))(i)
+    alt((
+        value("".to_string(), tag("\"\"")),
+        delimited(char('"'), parse_string_internal, char('"')),
+    ))(i)
 }
 
 fn string(i: &str) -> IResult<&str, builder::Term, Error> {
@@ -1195,6 +1198,11 @@ mod tests {
         );
     }
 
+    #[test]
+    fn empty_string() {
+        assert_eq!(super::string("\"\""), Ok(("", builder::string(""))));
+    }
+
     #[test]
     fn integer() {
         assert_eq!(super::integer("123"), Ok(("", builder::int(123))));