Fix field_metadata.is_series_index that apparently never worked.
Bug #1247348. This change might change behavior of something. Before, any int or float that was None or zero returned an empty string. Now only None values do that. The previous behavior did not work properly with zero values in templates, including series indices.
Template language: Fix zero valued series indices not formatting correctly

Fixes #1247348 [Template does not show correct result with series_index[0]](https://bugs.launchpad.net/calibre/+bug/1247348)

Merge branch 'master' of https://github.com/cbhaley/calibre