Handle empty default value during variable expansion (#1)

* Handle empty default value during variable expansion

* Make expression after colon optional

As suggested by @maxbrunsfeld

corpus/expressions.txt

@@ -28,12 +28,17 @@ Variable expansions
 
 echo ${abc}
 echo ${abc:-def}
+echo ${abc:- }
+echo ${abc:
+}
 
 ---
 
 (program
   (command (command_name (word)) (expansion (variable_name)))
-  (command (command_name (word)) (expansion (variable_name) (word))))
+  (command (command_name (word)) (expansion (variable_name) (word)))
+  (command (command_name (word)) (expansion (variable_name)))
+  (command (command_name (word)) (expansion (variable_name))))
 
 ===================================
 Other variable expansion operators

grammar.js

@@ -295,7 +295,7 @@ module.exports = grammar({
         seq(
           $._variable_name,
           choice(':', ':?', '=', ':-', '%', '/'),
-          $._expression
+          optional($._expression)
         )
       ),
       '}'

src/grammar.json

@@ -1083,9 +1083,17 @@
                     }
                   ]
                 },
+                {
+                  "type": "CHOICE",
+                  "members": [
                     {
                       "type": "SYMBOL",
                       "name": "_expression"
+                    },
+                    {
+                      "type": "BLANK"
+                    }
+                  ]
                 }
               ]
             }