NVMeMetric: MetricStore constructor allows empty Metric

The MetricStore now can accept empty metric from nvmed and nvmed
plugins, by ignoring these empty metrics.

The purposes are:
* prevent a daemon crash from an ill-written plugin
* allows the MetricStore owner pushes empty metric intentionally

Signed-off-by: Hao Jiang <jianghao@google.com>
Change-Id: I1f47c8042e3b2b8004632cc7058f38085aa08e53

src/NVMeMetricStore.hpp

diff --git a/src/NVMeMetricStore.hpp b/src/NVMeMetricStore.hpp
index 1cb84b3..607108b 100644
--- a/src/NVMeMetricStore.hpp
+++ b/src/NVMeMetricStore.hpp
@@ -55,13 +55,18 @@
         sdbusplus::xyz::openbmc_project::NVMe::server::MetricStore(
             dynamic_cast<sdbusplus::bus_t&>(*conn), objectPath.c_str()),
         path(objectPath), io(io),
-        metricStore{
-            {std::string{metrics->getIdentifier()}, std::move(metrics)}...},
+        metricStore{{std::string{metrics ? metrics->getIdentifier() : ""},
+                     std::move(metrics)}...},
         validFlag(std::make_shared<bool>())
     {
+        metricStore.erase("");
         auto vuMetrics = getVendorMatrics(path);
         for (auto& metric : vuMetrics)
         {
+            if (!metric)
+            {
+                continue;
+            }
             auto [itr, res] = metricStore.try_emplace(
                 std::string{metric->getIdentifier()}, std::move(metric));
             if (!res)