block_html configdata format may not be available

In block_html\search\content::get_document we assume that block instances configdata's format is always there, it seems that this is not the case (https://tracker.moodle.org/browse/MDL-53226?focusedCommentId=573681&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-573681)