Prevent crash when cgroups_ref is null in streamEntryIsReferenced() after reload (#14276)

This bug was introduced by https://github.com/redis/redis/pull/14130
found by @oranagra

### Summary

Because `s->cgroup_ref` is created at runtime the first time a consumer
group is linked with a message, but it is not released when all
references are removed.

However, after `debug reload` or restart, if the PEL is empty (meaning
no consumer group is referencing any message), `s->cgroup_ref` will not
be recreated.

As a result, when executing XADD or XTRIM with `ACKED` option and
checking whether a message that is being read but has not been ACKed can
be deleted, the cgroup_ref being NULL will cause a crash.

### Code Path
```
xaddCommand -> streamTrim -> streamEntryIsReferenced
```

### Solution

Check if `s->cgroup_ref` is NULL in streamEntryIsReferenced().

src/t_stream.c

@@ -2705,6 +2705,7 @@ int streamEntryIsReferenced(stream *s, streamID *id) {
         return 1;
 
     /* Check if the message is in any consumer group's PEL */
+    if (!s->cgroups_ref) return 0;
     unsigned char buf[sizeof(streamID)];
     streamEncodeID(buf, id);
     return raxFind(s->cgroups_ref, buf, sizeof(streamID), NULL);

tests/unit/type/stream.tcl

@@ -244,6 +244,33 @@ start_server {
         assert {[r XLEN mystream] == 1} ;# Successfully trimmed to 1 entries
     }
 
+    test {XADD with ACKED option doesn't crash after DEBUG RELOAD} {
+        r DEL mystream
+        r XADD mystream 1-0 f v
+
+        # Create a consumer group and read one message
+        r XGROUP CREATE mystream mygroup 0
+        set records [r XREADGROUP GROUP mygroup consumer1 COUNT 1 STREAMS mystream >]
+        assert_equal [lindex [r XPENDING mystream mygroup] 0] 1
+
+        # After reload, the reference relationship between consumer groups and messages
+        # is correctly rebuilt, so the previously read but unacked message still cannot be deleted.
+        r DEBUG RELOAD
+        r XADD mystream MAXLEN = 1 ACKED 2-0 f v
+        assert_equal [r XLEN mystream] 2
+
+        # Acknowledge the read message so the PEL becomes empty
+        r XACK mystream mygroup [lindex [lindex [lindex [lindex $records 0] 1] 0] 0]
+        assert {[lindex [r XPENDING mystream mygroup] 0] == 0}
+
+        # After reload, since PEL is empty, no cgroup references will be recreated.
+        r DEBUG RELOAD
+
+        # ACKED option should work correctly even without cgroup references.
+        r XADD mystream MAXLEN = 1 ACKED 3-0 f v
+        assert_equal [r XLEN mystream] 2
+    } {} {needs:debug}
+
     test {XADD with MAXLEN option and DELREF option} {
         r DEL mystream
         r XADD mystream 1-0 f v