Fix empty triple quote parse error

Author: metawilm

Diff for: parser/lexer.lisp

@@ -585,7 +585,9 @@ Used by compiler to generate 'forbidden' identfiers.")
                                                                       (not prev-bs))
                                           until (and (char= z x y ch1) (not prev-bs))
                                           finally (return (- %lex-last-read-char-ix% 3)))))
-                            (lex-substring start end)))
+                            (if (> start end)
+                                (return-from read-kind "")
+                              (lex-substring start end))))
 
                          ((char= ch1 ch2) ;; "" or '' but not """ or '''
                           (when ch3 (lex-unread-char ch3))

Diff for: test/parser-test.lisp

@@ -57,7 +57,12 @@
     (test-equal '([module-stmt] ([suite-stmt] (([literal-expr] :number 42)))) (ps "42"))
     (test-equal '([module-stmt] ([suite-stmt] (([literal-expr] :string "x")))) (ps "'x'"))
     (test-equal '([module-stmt] ([suite-stmt] (([literal-expr] :bytes "x")))) (ps "b'x'"))
-      
+    (test-equal '([module-stmt] ([suite-stmt] (([literal-expr] :string "")))) (ps "\"\"\"\"\"\""))
+    (test-equal '([module-stmt] ([suite-stmt] (([literal-expr] :string "")))) (ps "''''''"))
+    (test-equal '([module-stmt] ([suite-stmt] (([literal-expr] :string "x")))) (ps "'''x'''"))
+    (test-equal '([module-stmt] ([suite-stmt] (([literal-expr] :string " ")))) (ps "''' '''"))
+    (test-equal '([module-stmt] ([suite-stmt] (([literal-expr] :string "'")))) (ps "'''\\''''"))
+
     ;; variables
     (test-equal '([assign-stmt] ([literal-expr] :number 3) (([identifier-expr] {y} ))) (ps "y = 3" t))
     (test-equal '([assign-stmt] ([literal-expr] :number 3) (([identifier-expr] {len}))) (ps "len = 3" t))