psu-ng: Skip power supply validation if none are present

If no power supplies are present, it means the D-Bus objects have not
been created yet, so skip the power supply validation (the validation
may had been requested if the BMC rebooted while the host is on, so when
the psu monitor starts up, the power state changes to On and the
validation is requested).
The validation will be requested again when the power supply D-Bus
objects appear on D-Bus via the interfaces added callback.

Tested: Rebooted the BMC at host runtime and verified no errors were
created.

Change-Id: Ifc59d1add04163afb46a589c0b8387bb3e8636cf
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/phosphor-power-supply/psu_manager.cpp b/phosphor-power-supply/psu_manager.cpp
index 03ee9e4..a8d0d55 100644
--- a/phosphor-power-supply/psu_manager.cpp
+++ b/phosphor-power-supply/psu_manager.cpp
@@ -679,7 +679,7 @@
 
 void PSUManager::validateConfig()
 {
-    if (!runValidateConfig || supportedConfigs.empty())
+    if (!runValidateConfig || supportedConfigs.empty() || psus.empty())
     {
         return;
     }