Be tolerant of devices that don't report serials.

The USB spec explicitly says this is optional, so we shouldn't be
relying on it.

Bug: http://b/20883914
Change-Id: Icf38405b00275199bcf51a70c47d428ae7264f2b
diff --git a/adb/transport.cpp b/adb/transport.cpp
index bb5be6b..29cf580 100644
--- a/adb/transport.cpp
+++ b/adb/transport.cpp
@@ -853,7 +853,8 @@
 
 #if ADB_HOST
 
-static void append_transport_info(std::string* result, const char* key, const char* value, bool sanitize) {
+static void append_transport_info(std::string* result, const char* key,
+                                  const char* value, bool sanitize) {
     if (value == nullptr || *value == '\0') {
         return;
     }
@@ -869,7 +870,7 @@
 static void append_transport(atransport* t, std::string* result, bool long_listing) {
     const char* serial = t->serial;
     if (!serial || !serial[0]) {
-        serial = "????????????";
+        serial = "(no serial number)";
     }
 
     if (!long_listing) {