json reader: correctly handle empty objects in objects

Attempting to parse { "foo" : {} } previously resulted in an error.
(close-hash) expected to consolidate an object into the values of the
containing object (in the first line of code).  While there is none in
the case of the empty hash, the structure of the accumulator looked like
it contained an unconsolidated object, so it folded the values into the
keys and got very confused.

basis/json/reader/reader-tests.factor

@@ -1,4 +1,4 @@
-USING: arrays json.reader kernel strings tools.test
+USING: assocs arrays json.reader kernel strings tools.test
 hashtables json ;
 IN: json.reader.tests
 
@@ -58,3 +58,6 @@ IN: json.reader.tests
 { 0 } [ "      0" json> ] unit-test
 { 0 } [ "0      " json> ] unit-test
 { 0 } [ "   0   " json> ] unit-test
+
+! empty objects are allowed as values in objects
+{ H{ { "foo" H{ } } } } [ "{ \"foo\" : {}}" json> ] unit-test

basis/json/reader/reader.factor

@@ -15,7 +15,7 @@ IN: json.reader
     ] dip ;
 
 DEFER: j-string
-    
+
 : convert-string ( str -- str )
     read1
     {
@@ -30,17 +30,17 @@ DEFER: j-string
     dup
     [ 1string append j-string append ]
     [ drop ] if ;
-    
+
 : j-string ( -- str )
     "\\\"" read-until CHAR: \" =
     [ convert-string ] unless ;
-    
+
 : second-last ( seq -- second-last )
     [ length 2 - ] keep nth ; inline
 
 : third-last ( seq -- third-last )
     [ length 3 - ] keep nth ; inline
-    
+
 : last2 ( seq -- second-last last )
     [ second-last ] [ last ] bi ; inline
 
@@ -68,12 +68,12 @@ DEFER: j-string
     dup pop >array over push ;
 
 : (close-hash) ( accum -- accum' )
-    dup length 3 >= [ v-over-push ] when
-    dup dup [ pop ] dip pop swap
+    dup [ length 3 >= ] [ last V{ } = not ] bi@ and [ v-over-push ] when
+    dup dup [ pop ] bi@ swap
     zip H{ } assoc-clone-like over push ;
-                                                 
+
 : scan ( accum char -- accum )
-    ! 2dup . . ! Great for debug...
+    ! 2dup 1string swap . . ! Great for debug...
     [
         {
             { CHAR: \" [ j-string over push ] }
@@ -91,13 +91,13 @@ DEFER: j-string
             { CHAR: f  [ 4 read drop f over push ] }
             { CHAR: n  [ 3 read drop json-null over push ] }
             [ value [ over push ] dip [ scan ] when*  ]
-        } case 
+        } case
     ] when* ;
 
 : (json-parser>) ( string -- object )
     [ V{ } clone [ read1 dup ] [ scan ] while drop first ] with-string-reader ;
-    
+
 PRIVATE>
-    
+
 : json> ( string -- object )
     (json-parser>) ;