Fix warning on nil tag

Signed-off-by: Colin Leroy-Mira <colin.leroy-mira@unabiz.com>

Author: cleroymira-unabiz

lib/fluent/plugin/prometheus/placeholder_expander.rb

@@ -39,6 +39,8 @@ def build(placeholder_values)
         private
 
         def build_tag(tag)
+          return [] if tag.nil?
+
           tags = tag.split('.')
 
           placeholders = { '${tag}' => tag }