fix: reduce/foreach state variable should not be reset each iteration (#3205)

When the UPDATE query in a reduce or foreach syntax does not emit
a value, it should simply skip updating the state variable. But
currently it resets the state variable to null.

tests/jq.test

@@ -322,6 +322,10 @@ null
 [{"a":1}, {"b":2}, {"a":3, "b":4}]
 [-1, -1, -4]
 
+[foreach range(5) as $x (0; .+$x | select($x!=2); [$x,.])]
+null
+[[0,0],[1,1],[3,4],[4,8]]
+
 [limit(3; .[])]
 [11,22,33,44,55,66,77,88,99]
 [11,22,33]
@@ -825,6 +829,10 @@ reduce . as $n (.; .)
 null
 null
 
+reduce range(5) as $x (0; .+$x | select($x!=2))
+null
+8
+
 # Destructuring
 . as {$a, b: [$c, {$d}]} | [$a, $c, $d]
 {"a":1, "b":[2,{"d":3}]}