[Test Runner] Adds NullPointerException to match_error

Author: picandocodigo

elasticsearch-api/api-spec-testing/rspec_matchers.rb

@@ -323,6 +323,8 @@ def compare_string_response(expected_string, response)
     when /error parsing field/
       message =~ /\[400\]/ ||
         actual_error.is_a?(Elastic::Transport::Transport::Errors::BadRequest)
+    when /NullPointerException/
+      message =~ /\[400\]/
     else
       message =~ /#{expected_error}/
     end