Fix checking if tablets should be used in scyllaConnPicker Pick

Previously the check was always based on p.conns[0], but there
is a posibility that it is nil and it should be checked.
The check should be based on not nil connection
This commit introduces searching for first not nil connection
and checking tablet-awareness based on this connection.

scylla.go

@@ -370,24 +370,33 @@ func (p *scyllaConnPicker) Pick(t token, keyspace string, table string) *Conn {
 
 	idx := -1
 
-	p.conns[0].mu.Lock()
-	if p.conns[0].tabletsRoutingV1 {
-		tablets := p.conns[0].session.getTablets()
-
-		// Search for tablets with Keyspace and Table from the Query
-		l, r := findTablets(tablets, keyspace, table)
-
-		if l != -1 {
-			tablet := findTabletForToken(tablets, mmt, l, r)
+	for _, conn := range p.conns {
+		if conn == nil {
+			continue
+		}
 
-			for _, replica := range tablet.replicas {
-				if replica.hostId.String() == p.hostId {
-					idx = replica.shardId
+		conn.mu.Lock()
+		if conn.tabletsRoutingV1 {
+			tablets := conn.session.getTablets()
+
+			// Search for tablets with Keyspace and Table from the Query
+			l, r := findTablets(tablets, keyspace, table)
+
+			if l != -1 {
+				tablet := findTabletForToken(tablets, mmt, l, r)
+
+				for _, replica := range tablet.replicas {
+					if replica.hostId.String() == p.hostId {
+						idx = replica.shardId
+					}
 				}
 			}
 		}
+		conn.mu.Unlock()
+
+		break
 	}
-	p.conns[0].mu.Unlock()
+
 	if idx == -1 {
 		idx = p.shardOf(mmt)
 	}