Fix -netinfo backward compat with getpeerinfo pre-v26

CLI -netinfo will currently break when calling it on a node that is running
pre-v26 bitcoind, as `getpeerinfo` doesn't yet return a transport_protocol_type
field.

Fix this by adding an `IsNull()` check as already done for other fields, and also:

- avoid checking for the full string "detecting", and instead do the cheaper
  check for the most frequent case of the string starting with "v"

- drop displaying the "v" prefix in all the rows, as it doesn't add useful
  information, and instead use "v" for the column header

- display nothing during peer setup, like for the -netinfo mping and ping columns

src/bitcoin-cli.cpp

@@ -518,7 +518,7 @@ class NetinfoRequestHandler : public BaseRequestHandler
                 const std::string addr{peer["addr"].get_str()};
                 const std::string age{conn_time == 0 ? "" : ToString((time_now - conn_time) / 60)};
                 const std::string sub_version{peer["subver"].get_str()};
-                const std::string transport{peer["transport_protocol_type"].get_str()};
+                const std::string transport{peer["transport_protocol_type"].isNull() ? "v1" : peer["transport_protocol_type"].get_str()};
                 const bool is_addr_relay_enabled{peer["addr_relay_enabled"].isNull() ? false : peer["addr_relay_enabled"].get_bool()};
                 const bool is_bip152_hb_from{peer["bip152_hb_from"].get_bool()};
                 const bool is_bip152_hb_to{peer["bip152_hb_to"].get_bool()};
@@ -538,7 +538,7 @@ class NetinfoRequestHandler : public BaseRequestHandler
         // Report detailed peer connections list sorted by direction and minimum ping time.
         if (DetailsRequested() && !m_peers.empty()) {
             std::sort(m_peers.begin(), m_peers.end());
-            result += strprintf("<->   type   net tp  mping   ping send recv  txn  blk  hb %*s%*s%*s ",
+            result += strprintf("<->   type   net  v  mping   ping send recv  txn  blk  hb %*s%*s%*s ",
                                 m_max_addr_processed_length, "addrp",
                                 m_max_addr_rate_limited_length, "addrl",
                                 m_max_age_length, "age");
@@ -551,7 +551,7 @@ class NetinfoRequestHandler : public BaseRequestHandler
                     peer.is_outbound ? "out" : "in",
                     ConnectionTypeForNetinfo(peer.conn_type),
                     peer.network,
-                    peer.transport_protocol_type == "detecting" ? "*" : peer.transport_protocol_type,
+                    peer.transport_protocol_type.starts_with('v') ? peer.transport_protocol_type[1] : ' ',
                     PingTimeToString(peer.min_ping),
                     PingTimeToString(peer.ping),
                     peer.last_send ? ToString(time_now - peer.last_send) : "",
@@ -661,7 +661,7 @@ class NetinfoRequestHandler : public BaseRequestHandler
         "           \"feeler\" - short-lived connection for testing addresses\n"
         "           \"addr\"   - address fetch; short-lived connection for requesting addresses\n"
         "  net      Network the peer connected through (\"ipv4\", \"ipv6\", \"onion\", \"i2p\", \"cjdns\", or \"npr\" (not publicly routable))\n"
-        "  tp       Transport protocol used for the connection (\"v1\", \"v2\" or \"*\" if detecting)\n"
+        "  v        Version of transport protocol used for the connection\n"
         "  mping    Minimum observed ping time, in milliseconds (ms)\n"
         "  ping     Last observed ping time, in milliseconds (ms)\n"
         "  send     Time since last message sent to the peer, in seconds\n"