Bug fix in [cfdb-html] variable substitution when data for column is not present.

git-svn-id: https://plugins.svn.wordpress.org/contact-form-7-to-database-extension/trunk@1024681 b8457f37-d9ea-0310-8a92-e5e31aec5664
Stable tag: 2.8.17

git-svn-id: https://plugins.svn.wordpress.org/contact-form-7-to-database-extension/trunk@1024683 b8457f37-d9ea-0310-8a92-e5e31aec5664