Revert "do not include bind relevant with empty string, resolves #34"

This is no longer necessary as we simply need to remove whitespace from
cell values in xls2json_backends

This reverts commit fae601cd269c5c40150bb080b3202f44a994de11.
remove whitespace from cell values [begin and end], results in a clean workbook dict with no unnecessary fields e.g whitespaced bind:relevant/bind:constraint