TT#149550 do not try to load preferences for an empty uuid

Change-Id: I0e76d1f503a532df2a60c225c63fda783c4cb081
(cherry picked from commit 9d4800ceda)

ngcp/pref.lua

@@ -131,14 +131,14 @@ function NGCPPrefs:_load(level, uuid)
 end
 
 function NGCPPrefs:caller_load(uuid)
-    if not uuid then
+    if not uuid or uuid == '' then
         return {}
     end
     return self:_load("caller", uuid)
 end
 
 function NGCPPrefs:callee_load(uuid)
-    if not uuid then
+    if not uuid or uuid == '' then
         return {}
     end
     return self:_load("callee", uuid)

tests/ngcp_pp.lua

@@ -96,12 +96,18 @@ TestNGCPPeerPrefs = {} --class
 
     function TestNGCPPeerPrefs:test_caller_load_empty()
         lu.assertEvalToTrue(self.d.config)
+        mc:replay()
         lu.assertEquals(self.d:caller_load(), {})
+        lu.assertEquals(self.d:caller_load(''), {})
+        mc:verify()
     end
 
     function TestNGCPPeerPrefs:test_callee_load_empty()
         lu.assertEvalToTrue(self.d.config)
+        mc:replay()
         lu.assertEquals(self.d:callee_load(), {})
+        lu.assertEquals(self.d:callee_load(''), {})
+        mc:verify()
     end
 
     function TestNGCPPeerPrefs:test_caller_load()