Make reference operators more consistent

* geesh/lexer.scm (get-parameter-expression): Use '#f' to signal an
omitted optional word instead of just omitting the word in the
resulting form.
* tests/lexer.scm: Add a test for omitted optional words.

geesh/lexer.scm

@@ -296,7 +296,7 @@ leading '$')."
               (match (get-char port)
                 (#\} (match `(,parameter ,operator ,word)
                        ((p #f #f) `(<sh-ref> ,p))
-                       ((p o #f)  `(,o ,p))
+                       ((p o #f)  `(,o ,p #f))
                        ((p o w)   `(,o ,p ,w)))))))))))
 
 (define (get-parameter-expansion port)

tests/lexer.scm

@@ -161,6 +161,10 @@
    ("#" . <sh-ref-skip-min>)
    ("##" . <sh-ref-skip-max>)))
 
+(test-equal "Recognizes a parameter expansion operator without default"
+  '((WORD (0 . 7) (<sh-ref-or> "foo" #f)))
+  (tokenize "${foo-}"))
+
 (test-equal "Splits multidigit parameter name without braces"
   '((WORD (0 . 3) ((<sh-ref> "1") "2")))
   (tokenize "$12"))