bmc: check empty instead of zero size

Minor tweak to call the empty() method on the container instead of
checking the result of the size() method.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: Ib5ee66c724783c4a5f63d974032f107b88264375
diff --git a/bmc/firmware_handler.cpp b/bmc/firmware_handler.cpp
index 4d06b58..b995671 100644
--- a/bmc/firmware_handler.cpp
+++ b/bmc/firmware_handler.cpp
@@ -48,7 +48,7 @@
         log<level::ERR>("Must provide at least two firmware handlers.");
         return nullptr;
     }
-    if (!transports.size())
+    if (transports.empty())
     {
         return nullptr;
     }