fix(plugins.js) : makes the config parameter 'withHeadings' works when data is empty

[loic-bellinger]

Noticed that the config parameter withHeadings was not properly working.

More precisely:

table: {
  class: Table,
  inlineToolbar: true,
  config: {
    withHeadings: true
  }
}

was not creating an empty table with headings.

So I made a fix. Tried to change as little code as possible (but I believe getConfig should be optimized further)

This PR https://github.com/editor-js/table/pull/126 also tackles the issue