Certificate manager: Fix application crash for empty units

During upload/delete certificates certificate manager does
service restart/reload based on "unit" parameter provided
during build time. This is an optional input to support
service related action from certificate manager application.
In the existing delete interface not added any check related
to this and causing application crash due to empty unit
input systemd function.

Proposed fix is to add empty check before consuming unit variable.

Change-Id: I2b0b2bf1fab1db32a00c1c96333388be2af99cc7
Signed-off-by: Jayanth Othayoth <ojayanth@in.ibm.com>
diff --git a/certs_manager.cpp b/certs_manager.cpp
index 3a530c4..d8c6263 100644
--- a/certs_manager.cpp
+++ b/certs_manager.cpp
@@ -347,7 +347,7 @@
             log<level::INFO>("Certificate file not found!",
                              entry("PATH=%s", certPath.c_str()));
         }
-        else
+        else if (!unit.empty())
         {
             reloadOrReset(unit);
         }