Fix samples metadata processing

In this patch we are fix metadata processing for samples with
unexpected metadata.

Change-Id: Ie95967c6b0700e5d0777dd236d1824a148f9f276

deployment_scripts/puppet/modules/telemetry/files/common/samples.lua

@@ -100,6 +100,9 @@ function get_field(field, metadata)
         if type(field) == 'table' then
             value = metadata
             for _, field_part in ipairs(field) do
+                if not value then
+                    break
+                end
                 value = value[field_part]
             end
         else