Fix lexing of empty double quotes

* geesh/lexer.scm (get-double-quotation): Handle empty quotation.
* tests/lexer.scm: Add a test for this and for empty single
quotes (which already worked correctly).

geesh/lexer.scm

@@ -385,6 +385,7 @@ next character statisfies @var{pred} (or is a newline)."
          (#\" (begin
                 (get-char port)
                 `(<sh-quote> ,(match (join-contiguous-strings (reverse! acc))
+                                (() "")
                                 ((word) word)
                                 (words words)))))
          ((or #\$ #\`)

tests/lexer.scm

@@ -225,6 +225,10 @@
   '((WORD (0 . 12) (<sh-quote> "foo\n#\\`$<\"")))
   (tokenize "'foo\n#\\`$<\"'"))
 
+(test-equal "Lexes an empty single quotation"
+  '((WORD (0 . 2) (<sh-quote> "")))
+  (tokenize "''"))
+
 ;;;
 ;;; Double quotations.
 ;;;
@@ -241,6 +245,10 @@
   '((WORD (0 . 9) (<sh-quote> "foo\n#<'")))
   (tokenize "\"foo\n#<'\""))
 
+(test-equal "Recognizes an empty double quotation"
+  '((WORD (0 . 2) (<sh-quote> "")))
+  (tokenize "\"\""))
+
 (test-equal "Respects escapes for special characters in double quotations"
   '((WORD (0 . 10) (<sh-quote> ("foo" (<sh-quote> "\"") "bar"))))
   (tokenize "\"foo\\\"bar\""))