Blocking XREAD[GROUP] should always reply with valid data (or timeout)

This commit solves the following bug:
127.0.0.1:6379> XGROUP CREATE x grp $ MKSTREAM
OK
127.0.0.1:6379> XADD x 666 f v
"666-0"
127.0.0.1:6379> XREADGROUP GROUP grp Alice BLOCK 0 STREAMS x >
1) 1) "x"
   2) 1) 1) "666-0"
         2) 1) "f"
            2) "v"
127.0.0.1:6379> XADD x 667 f v
"667-0"
127.0.0.1:6379> XDEL x 667
(integer) 1
127.0.0.1:6379> XREADGROUP GROUP grp Alice BLOCK 0 STREAMS x >
1) 1) "x"
   2) (empty array)

The root cause is that we use s->last_id in streamCompareID
while we should use the last *valid* ID

tests/unit/type/stream-cgroups.tcl

@@ -147,6 +147,20 @@ start_server {
         assert {[lindex $res 0 1 1] == {2-0 {field1 B}}}
     }
 
+    test {Blocking XREADGROUP will not reply with an empty array} {
+        r del mystream
+        r XGROUP CREATE mystream mygroup $ MKSTREAM
+        r XADD mystream 666 f v
+        set res [r XREADGROUP GROUP mygroup Alice BLOCK 10 STREAMS mystream ">"]
+        assert {[lindex $res 0 1 0] == {666-0 {f v}}}
+        r XADD mystream 667 f2 v2
+        r XDEL mystream 667
+        set rd [redis_deferring_client]
+        $rd XREADGROUP GROUP mygroup Alice BLOCK 10 STREAMS mystream ">"
+        after 20
+        assert {[$rd read] == {}} ;# before the fix, client didn't even block, but was served synchronously with {mystream {}}
+    }
+
     test {XCLAIM can claim PEL items from another consumer} {
         # Add 3 items into the stream, and create a consumer group
         r del mystream

tests/unit/type/stream.tcl

@@ -191,6 +191,17 @@ start_server {
         assert {[lindex $res 0 1 0 1] eq {old abcd1234}}
     }
 
+    test {Blocking XREAD will not reply with an empty array} {
+        r del s1
+        r XADD s1 666 f v
+        r XADD s1 667 f2 v2
+        r XDEL s1 667
+        set rd [redis_deferring_client]
+        $rd XREAD BLOCK 10 STREAMS s1 666
+        after 20
+        assert {[$rd read] == {}} ;# before the fix, client didn't even block, but was served synchronously with {s1 {}}
+    }
+
     test "XREAD: XADD + DEL should not awake client" {
         set rd [redis_deferring_client]
         r del s1