Fixed issue with displaying empty triggers

It was not possible to display empty triggers due to one of the property
handlers returning error. This fixes handlers and proper value is
returned instead.

Tested:
- All unit tests are passing.
- Can introspect empty triggers
- Empty triggers are correctly listed in redfish

Signed-off-by: Krzysztof Grobelny <krzysztof.grobelny@intel.com>
Change-Id: I1ce9a9a43faac5446911f0e44c638b4ca91adb82
diff --git a/src/utils/conversion_trigger.cpp b/src/utils/conversion_trigger.cpp
index e1b835a..bb0c3bc 100644
--- a/src/utils/conversion_trigger.cpp
+++ b/src/utils/conversion_trigger.cpp
@@ -82,6 +82,11 @@
     fromLabeledThresholdParam(const std::vector<LabeledThresholdParam>& params)
 {
     namespace ts = utils::tstring;
+    if (params.empty())
+    {
+        return std::vector<numeric::ThresholdParam>();
+    }
+
     if (isFirstElementOfType<std::monostate>(params))
     {
         return std::vector<discrete::ThresholdParam>();